GM-GMC: 93 GMC 1/2 ton extended cab pickup rear light problem, lighting issues, wiring connector

Question
My dad drives the above pickup and about every week or two the rear backup (reverse), brake and parking lights go out.  The only rear lights that work are the turn signals.  The only way I found to temporarily fix the problem is to unplug and plug back in the 4 wire harness connector located between the rear bumper and the spare tire.  Once doing so the lights function normally for about a week or so.  The connector and wires look perfect, truck has around 60K miles on it, always garaged and I'm really stumped. Is there some sort of Module that could be going bad?

Answer
A good place to start with any rear lighting issues is grounds.  Check the frame ground which should be located at the back near the connector that you refered to.  Next check your trailer wiring, it is VERY common to have a green or corroded trailer wiring connector cause the symptoms you are describing.