I've been following the progress on this.
You folks looking for a persistent 12v power source in the rear of the car can refer to the instructions for wiring the Lighted Rear Applique'. This may be the same connection that AMPUP was referring to in his post #40.
If you are not familiar with the Lighted Rear Applique, read about it here
http://evappliques.com The instructions link is located here:
EV-Appliqués - Installation and here
https://docs.google.com/document/d/1a-G4Ry1lO81c65rj8hGadJ-IJBQBlQHfSrS-...
In the Gen III Telsa Model S Lighted Rear Applique' instructions go down to the top of page 17 with paragraph titled 12V Power:
You might want to consider adding a fuse panel or small fuse box coming off the liftgate control module connector and getting power from the fuse panel. This will allow a graceful 12v power connection for other additional devices you might want to add later - in the trunk.
By the way, the Lighted Applique' is awesome.