On my '69, the rear wiring harness runs down the passenger frame rail. The harness split and ran across the rear crossmember to the drivers side. I'd spend some time looking at that wiring and the split that goes to the drivers side. I went ahead and put a junction block in the rear and...
Find it hard to believe that the divers & passenger brake lights are on separate circuits from the switch to the back of the truck. I would confirm this to be true before spending a bunch of time up at the switch, otherwise, I would be looking at other possibilities at the light or with the...