I recently purchased an '01 996 and was changing out a few bulbs just because the old ones were dim from age (built up "soot" on inside of bulbs). After replacing the backup lamps, after about 2 minutes they stopped working. I checked fuses and found no fault, found 12v at fuse. Checked the sockets and found 12v at center contact and ground continuity at side contact, both right and left side. Placed new bulbs into sockets, nothing still. Checked voltage at socket again. Found that when one bulb (either side) was inserted in socket, the other socket would no longer have a 12v reading. Doesn't matter which side, works the same way. No bulbs inserted, 12v reading. I've never seen anything like this before on any other car, and can find no logical explanation other than maybe the reversing lamp switch somehow became suddenly faulty. Other than that, I can't figure out what else could cause such a simple circuit to fail in this manner. Any and all help appreciated...

 

If you have full power to the sockets, it sounds like the wrong bulbs.

 

Thank you for the response. I have considered that perhaps the bulbs I used may have been of a lesser or greater wattage (electrical draw) than the original ones, so I did attempt to put those back in as well. No go... I can't imaging that as fine a car as this is that it's so incapable of powering a simple 12v bulb. Just to clarify something that wasn't mentioned in the original posting, the back up lamps did consistently illuminate prior to my bulb "swap".
Not fully understanding exactly how a 12v bulb for reversing lights can be "wrong", I headed to the dealer earlier today to explain the symptoms. The service advisor seemed equally perplexed, yet he too said it could be bad or "wrong" bulbs. I don't know how Porsche gets into using such "proprietary" bulbs, but in any case, I spent the few extra cents to procure two of their "special" reversing lamp bulbs (as well as a reversing lamp switch, one of Porsche's cheaper parts I'm sure). Upon returning home I got the magic beans out of the sack and mom threw them out the window in disgust, OOPS, sorry wrong story, I removed the new Porsche bulbs from their sealed sack and installed them, turned on the key and put the car in reverse. Guess what, NO ILLUMINATION. When the car is cool enough, perhaps tomorrow AM, I'll start working on replacing the switch...

 

...switch replaced, lights working. Can't believe that it was either A) coincidence or B) regular 12v automotive lamps would cause switch failure.
Note: I'm putting the "other" lamps back in and trying again. Since the switch is relatively cheap and the change out was relatively simple once the plastic cover was lowered, no biggie if I blow it again, but I can't believe that the "other" lamps would cause this... Thanks again...