Any idea why the instrument lights light up when the brakes are applied? I've been over/under/through this thing MULTIPLE times and cannot find a crossed wire in what appears to be the lighting wiring.
One step forward 10 back! Haha
Does the truck have a trailer plug? My wagon has one. One of the times I actually used it, the wiring on the trailer was bad, so, each time I hit the brakes, my dome light came on....
The harness on the trailer was pinched between its fender and frame rubbing the wiring bare.
Maybe something similar is going on with yours (minus the trailer)? Something not spliced together correctly, or a spliced joint failing?
Just a WAG.....