I'll try to keep this short.When I got my 1980 F100,the reverse lights didn't work.I had some good new bulbs layin around,so I tried them out.They didn't work.So I gave up cause it wasn't important to me.Well fast forward to yesterday.After learning a little more about the wiring on my truck over the last few months,I set out to clean the NSS on the transmission to see if that was my problem.It was covered in grease,and after a good cleaning,I put it back on.I hopped in the truck and tested the reverse lights.The passenger side worked,but the driver's side didn't.I checked the bulb,but nothing was wrong with it.I swapped another one in just in case,but nothing.I proceeded to look for any shorts or ground problems.Nothing.I unplugged the rear harness from the main plug,and checked for voltage.It's getting plenty of voltage.I checked the reverse light socket for any damage,but there wasn't any.I then noticed that there was a clump of wires all taped together in some kind of adhesive backed black cloth.I pulled the cloth off,and found 4 wires twisted together.I don't think this is Factory,and I have a feeling that it is what's keeping my reverse light from coming on.I tried looking at my hayne's manual,but the wiring diagrams don't even list half the wires on my truck.Does anybody know of a site with good wiring diagrams?Or maybe someone has a good one at their house they can have a look at for me....Here's the wiring colors if it helps...

-Brown with pink stripe that goes to the harness connector near the gas tank.
-Solid black that also goes to the harness connector.
-Green with white stripe that goes to the reverse light socket
-Another solid black that goes to the ground on the bed.

All help is greatly appreciated.

 

Black with pink stripe is the reverse wire. If one of them is working, then the problem has to be at the rear, since there is a splice that divides the power to each side.

One quick rule; Try to do any voltage testing with the bulbs plugged in to load the circuit, or use a testlight that will load the circuit.