I noticed that 997.2 GT3's have a large cooling duct under the rear lower control arm and I wondered if they would fit my 997.1 Carrera S. You can order them from the dealer but Suncoast has a kit with all the parts.
Here is what you do;
Jack up and support the car properly
Remove the rear wheel
Remove the rear caliper and hang it out of the way, be carefull of the wireing attached to the rear of the caliper.
Remove brake disc
Remove backing plate
Place bracket over the control arm and slide it to the stop on the control arm
Slide in one side of bracket to the duct
Use supplied screws or attach other side of bracket to duct, I actually used longer screws, just could not get the short ones to catch.
Measure and trim backing plate so air comming out of duct can hit disc.
Re attach backing plate, disc, brake caliper, and remount tire.
Hopefully this will extend the life of the brake discs.

I did wonder if this was a weird design, since all the air blows on only one side of the disc, but I believe Porsche must know best.

 

Spread the pads

 

Pry out the wire carefully

 

Fitting duct

 

Line everything up and insert screws

 

It's really tight between the sway bar and rear axle

 

Not much metal needs to be removed, I used a pair of tin snips

 

Lime Rock Park this Thursday, let's see how they work.