After having retrofitted seat heating in to my E46 M3 http://www.m3torque.co.uk/showthread.php?t=1111 I got to liking seat heating during the winter months, so I started investigating what was involved in doing the same to my replacement daily driver. As a result I have just completed a seat heating (option M139 & M348) retrofit in my 996 Turbo with comfort seats, and thought I would share some of the technical points of the retrofit for others looking at doing the same.

The only comprehensive guide out there is Denis Vogel’s retrofit in to a Boxster http://dennisvogel.com/heatedseats/index.html . The 996 is essentially the same and Vogel has gone to a lot of trouble in putting together a good DIY website, however his retrofit involved making up a few non standard parts where I wanted mine to be as close to factory as possible. So rather than creating a new DIY guide from scratch my intention here is to add some more context to the existing knowledge base of Vogel and answer some of the questions that I had which Vogel’s guide didn’t answer for me. So here is my 2 cents extra to the existing body of knowledge. Before attempting this retrofit it would be virtually impossible to attempt this without a set of terminal removal tools. I bought a 23 piece set off eBay for about US$100.
As stated, I wanted my retrofit to be as close as possible to a factory fit, whilst this isn’t technically possible as the factory loom has the wiring incorporated in the main loom, I set about creating a retrofit loom. Initially I was going to get a loom made so I studied the wiring diagram:


I then set about trying to find a second hand main body harness loom off eBay for a 996 with the seat heating option. The way to tell if a loom you are looking at has the option is that it should have a fuse (which happens to be 25 amp) in the top row (row A) position 6. If it has a fuse it has seat heating, if it doesn’t then no heating. There weren’t very many for sale when I was looking and 996 looms were very expensive, prohibitively so. I then decided to compare the wiring diagram of the 996 to the 986 Boxster and found that the seat heating loom was identical including the wiring colours to the 996. The only difference (unconfirmed) is that the loom may be a little longer as the switches in the 986 are at the ashtray in the centre console instead of below the climate control panel in the 996. Knowing this made it easier to find a loom as there were many more Boxsters being broken for spares, thus the looms were much cheaper. I found a loom on Ebay in Germany and had it posted to Australia, I was now guaranteed to have an OEM loom that was pre-terminated with the right ends, in the right colours and at the right lengths answering all the unknowns to making your own loom. My loom turned up and I set about cutting out the wires I needed.



This is much easier than it sounds. Firstly you just needed to find the plugs that connect the main body loom to the seats (plugs X16 & X17 in technical speak) and work back from there. If you look at the diagram again these wires go back to the two switches, one wire to the A6 fuse for power and one other wire to a terminal in the relay box (BS15 in technical speak) which provides power for the backlight on the switches. Here is the main loom harness that connects to X16 and X17:





You need to start removing the pins from this plug as you will be inserting the wires in to your own X16 & X17 plug in your car. So undo the cable tie and fold back the cover by releasing the side clips and then start removing the terminals from the plug with your terminal removal tools. If you bought a loom like I did practice on other terminals of the same type first to you get the hang of doing it.

 

You need to release the round brown earth wire out of pin 6 and the power red~violet wire out of pin 3 that ultimately goes to the fuse box A6, take note to slide out the blue “fuse/wick” in the connector before pulling them out.

Now in the X16 / X17 plug on the side there is a plastic side cover that slides down the side of the connector to release the plugs inside it. Put a large screw driver in the gap at the top of this and lever off the plug twisting the screw driver and this side will slide off. Now you can release the seat heating wires which are grouped together in another plug inside of X16 / X17.

 

Once you’ve released the wires out of X16/X17 then start cutting the cloth tape back on your loom and follow the wires back to the switches, fuse box (A6) and light relay (BS15), BE CAREFUL NOT TO CUT YOUR WIRES IN THE PROCESS! Take note if you can wear some surgical gloves do so as the glue off the tape is terrible on your hands.

The fuse box – Terminal/Fuse A6.
The wire that goes to A6 is a red~violet wire and is the power to the seat heating relay that you will eventually be plugging in to under your seat. Firstly remove the fuse strip from the fuse carrier:

Remove the wick out of the fuse strip:


With your terminal removing tools remove the red~violet wire from the terminal housing. This is only one half of what the fuse plugs in to, on my car the other half (that the fuse joins) was prewired in to the fuse box.

Removing the Grey~Blue~Red Backlight wire

Now returning to the switch plug/connector at the centre console, follow the Grey~Blue~Red Backlight wire all the way to the back of the relay housing which is connector BS15:

 

The BS15 terminal point looks like this from the front and is located in position 11:




Slide the complete plug out of the housing and then remove the internals from the plug:



Once this is out you’ll need to get the internals out of it and then you will find that the terminal slides on to a multi-pin connector. It was difficult to get the red internals out of the grey plug without doing a bit of bruising to the plug. Once out though, it was obvious what you needed to do.

At this point you have removed all of the seat heating wires from the main loom. I then bought some factory cloth braded tape off eBay and bound my loom in a “Y” formation and guessed at what point the red~violet and Grey~Blue~Red wires would need to exit the loom. I did this by laying the loom out in the car and working out where it would sit. Take note here, the switch plugs which are mounted below the climate control module in the centre console are different colours, white is the left one and black is the right one. The left one has the Grey~Blue~Red wire going all the way back to BS15, then this wire jumps from the left (white) plug to the right (black) plug. Make sure when you lay the loom out in your car that you don't get the runs to the seat switched over the wrong way, or you'll find that the left switch operates the right seat and vice versa. Test your wires with a multimeter to make sure your white plug wires go to the left seat. If you get it wrong you can't just "swap the switches over" as they are handed. In this case you will need to pull the pins out of the plugs at the switches and swap them over, then swap the Grey~Blue~Red wire over to the other plug. You need the double Grey~Blue~Red wire to go to the white plug which then jumps to the black right plug at the batwing point where the switches are.

Once the loom was bound you now need to sit it in the car. So you need to remove your seats and the door sill covers. The door sill covers come off by removing two plastic plugs in the covers inside the car and then inserting an allen key in the hole. You don’t need to remove the allen key screws, just undo them and then pry the plastic sill cover up. This then allowed me to lift the carpet and lay my loom alongside the route of the main harness taping it with the cloth tape every so often and running it through the same cable tie points etc.
Insert your ends in to the X16/X17 plugs and then connect your power wire in to fuse A6 and the light power wire in to BS15. There is a lot of work in this, more than the sentence conveys! Then continue to run the loom to the centre console under the carpet up to the bat wing area where the switches are.

Seat Heating Earth point

At this point I should discuss the earth point in the car. Vogel mentions running an earth point in behind the centre console as he couldn’t find/get to the correct earth point. I am not sure why he did this as the earth point turned out to be a non-event. When you look at the wiring diagram it shows that the loom in the seat has two wires going in to the X16/X17 plug at pin 6. The other side of this plug is what initially concerned me as it shows the wires coming out of A6 and then going in to A4 which links it in to the main earth line for all of the seat, this then goes to an earth point under the carpet, ground points 6 and 7 as per the diagram below:


Without checking my car I thought that if I didn’t have seat heating then I wouldn’t have the small jumper in X16/X17 between pins A6 and A4 so I started to take out this earth loom and was going to swap it over. It looked to be straight forward on one side of the car, but the other side was going to entail uncrimping at an earth point as the existing lead in the car was crimped and soldered at a major junction block on the way to the ground points. The loom was looking like this, notice the double connected brown wire in the right hand of the picture. These are the connectors in pins A6 which jump back to A4 at the X16/X17 plug before making their way to the ground point:

 

However when I pulled my seats out much to my surprise and relief I found that the standard earth loom on my car already had the jumper on the car side of X16/X17 between pins A6 and A4, so I didn’t have to do anything.

Installing the loom and relay in the seat
The loom and relay in the seat was pretty straight forward and I won’t go in to details as to how to remove the seat covers as there is a good DIY here:
https://www.6speedonline.com/forums/...r-cushion.html

The loom you need to install already comes with the connectors and plugs so it is literally plug and play.

I followed the routes shown in the official guide. The relay is mounted on the outer-sides of the seat with the pins facing the front of the car. Slide your relay bracket in to the side of the relay and then screw the bracket in to the seat base at its predrilled locating point.

 

On the relay loom you will notice that there is a black plug that slides in to the X16/X17 plug once you remove the side cover. On the driver’s seat there is a white plug that sits next to it which holds it in place once the side cover is put back in place on the side of the plug. These are the two connectors that go in to the X16/X17 plugs and the side cover, the black plug shown below is already on the relay loom, you need to buy the white plug and slide lock:


This is how these connectors nestle together:



The passenger’s seat didn’t have the white connector or the side lock, so you will need to buy both of these otherwise the black connector just flops around in the X16/X17 connector. Nothing connects in to the white connector on the passenger’s seat, it is there to hold the black one in place.

Double check that your wire colours marry up once everything is fitted in the X16/X17 connectors.
Now that you’ve got everything done insert your 25amp fuse in to A6 in the footwell and reconnect your battery. Test it to see if you’ve done everything right:


SUCCESS!

And that is how you retrofit seat heating in to a 996. The part numbers I needed for this retrofit were:

· 996 521 531 01 – backrest cushion with heating x 2
· 996 521 525 00 – Seat cushion with heating x 2
· 996 618 521 00 – Seat heating relay x 2
· 900 143 191 02 – Screw to hold relay bracket in place x 2
· 996 612 540 01 – Seat heating wiring harness in seat x 2
· 996 618 531 00 – Seat heating relay bracket x 2
· 996 521 937 00 – Seat base leather cover clips to affix to seat base x 10
· 996 552 335 02 01C – Trim for seat heating switches in black plastic
· 996 613 153 10 A05 – Left hand seat heating switch (matt black 03-) white backed x 1
· 996 613 154 10 A05 – Right hand seat heating switch (matt black 03-) black backed x 1
· 999 650 218 40 – White connector that goes in to X16/X17 seat plug on passenger’s side x 1
· 999 652 960 40 – Slide side cover on X16/X17 plug on passenger’s side x 1 (also comes with a Boxster’s loom.)
· 999 607 087 00 – Standard 25 Amp fuse (pull one out of the Boxster loom)
Estimated cost between US$1800 and $2000. Thanks to the guys at Sonnen Porsche (www.porscheoemparts.com) for supplying the parts.

Sidebar:
I bought new foam with the seat heating elements stuck to the top of them. The new seat base foams (996 521 525 00) have a different profile on the side bolsters to the originals, they aren’t as high and taper off more as they cut down to the backrest, this causes the leather to be a bit more floppy at these points. The foams had “Targa” cast in them, but they are the only part number listed without you want the lowered seat option. However I am glad I did buy the new foams and not just the heating elements as after 65,000 kilometres my driver’s seat foam was split right through and was literally breaking down.

 

Holy ****!!!!!!!

That is amazing work - props for attempting that

 

Wow...what a tremendous and thorough post. Thanks for doing so, even though I have heated factory seats I can tell this is a big deal. Reps to you, and you should ask a moderator to move this into the 996TT knowledge base section.

 

Tenacious effort, thanks for posting. I too have heated seats, but wanted to recognize the effort.

 

thank you very much for the diy

 

This is impressive! Amazing work! I'm glad I already got the seat heating installed ;-)

 

Monumental effort would be an understatement!

 

WOW! Props to you for taking the time to do a nice write up! I just did the same thing in my GF's X3 and I think the connectors are identical. A lot of work to have your butt warm but it is worth it!

 

Thanks so much for this post!! I just did this for my new (to me) 2002 C4 Cab. It was so easy and I followed your advice and got a used harness off eBay for around $170. Was really easy pulling the wiring out of the harness and adding it to my car. I didn't have to buy the terminal kit as I just removed the terminals from the connectors with some brute force and if I broke a terminal from the harness I bought, no biggie since I don't need the old one. The connectors just snap easily into the existing connectors on your car.

I got used heated seat backs on eBay for around 90 bucks and the new seat bottoms for around 300. The wire harness and relay with bracket for both seats came as a set on eBay for around 250. The batwing with buttons came as a set too from eBay for around 50. It was a tedious job and took in all around 10-12 hours to complete. Under a 1000 for parts and my labor was well worth it to not have my wife complain about being cold when the top is down!

Thanks again for posting this as I couldn't have done it without this!!


Now if I could just find the wiring in the harness I bought for the Auto Dimming Mirror and Rain Sensor Wipers, I'd be golden. the wiring is so different for cars with and without it!! Thanks again!!

 

Glad to be of assistance! Thanks for posting back.