If you don't have an electrical diagram for your car try this link. If you are familar with Porsche electrical you should be able to trace the circuit for the 3 fuse panel in the back of the car.
http://www.pelicanparts.com/911/911_Parts/911_electrical_diagrams.htm
See if you can contact Targa80 he's pretty good with circuits. And he's a great guy when it comes to helping out fellow Porsche owner.