i'm Francesco and i've the same problem on my Cayenne 2004. But i've a DIy solution.
Sorry for my english.
- Buy a rear support for the heating rear seats
- inside the electronic regulator, instal a usb port
from the light of rear "air" get the electric signal to a rele'. when you switch on the Light (car) the releŔ go ON. on the same rele' use the 12 volt from the soket.
Buy a 12v- 5v double usb adapter.
Connect all and soldering to usb plug.
It's to hard to write, than do it