Agree with above - just gotta break out the multimeter and start chasing voltage until you find the spot it disappears.
One of the more common rear end electrical issues I see is water intrusion corroding the contacts inside of the electrical connectors behind the cargo panels. ANY wheel well rust hole will let enough water in, even a pinhole. Those connectors serve power to the tail lights, brake lights, license plate lights, rear blinkers, etc. I think they provide power to electric fuel pumps on EFI models as well, everything in the rear goes through those connectors. Once the corrosion starts, the rear end power stuff stops working slowly over time.
