[70GMCer]

I'm finding out that these trucks have grounds all over the place. So, I wouldn't be surprised if the fuse panel is grounded.

My left rear signal was not working & the backup lights also.

So I looked under the bed at the rear where the harness was, & low and behold, someone had really did a butcher job on that harness.

I took out the harness to inspect. I noticed that there is a ground strap within that harness that bolts to the area where the tail light lens bolts to that wasn't hooked up.

After re-soldering & repairing all the damaged wires, I did a continuity check on the harness before installing.

I also put die-electric grease on all of the connections including the main plug.

They get corroded over time & need cleaned. Do that to all of the connections throughout the truck.

All checked out & installed. All the lights now work including the backup.

I'm not saying this is your problem, but at least check that ground strap to make sure it is bolted to the bed (tail light housing opening).

Good luck as I am still looking for those little electrical "Gremlins" hiding....