Review this thread, it has most of what you need, in the end I took power for the rear camera from one of the front fuse panel on the passenger side.Sorry for your case. Works perfectly in my case with PCM 3.1 and waiting to connect to camera. Hope somebody answer my questions.
I wanted to do something about the poor rear vision on my '14 Cayman. This DIY describes what I did. The things you will need: NAV-TV PCM3.1-XG-CAM ($700) https://navtv.com/products/NTV-KIT375/pcm31-xg-cam.html Backup camera ($30) Amazon.com : COOLINT CT102G Light Gray Solid Zinc Alloy...