Nice chase light.
Don't know if this helps or not...
On the exterior, rear cab wall, in front of the right-rear tire, there is a three wire weather pack connector (WPC) in the OEM wiring harness.
I built a tail/stop jumper harness for a third brake light on the trunk box. I connected the brake light wires into the car at that WPC connection.
Looking straight into the female end of the WPC, with its locking lug to the left:
Top pole is ground
Middle pole is positive/tail
Bottom pole is positive/stop
The wire colors were not visible.