reverse lights on freightliner 1991 FLD112

Got pulled over by DOT For an inspection. everything checked out except one of my tail light bulbs, and my reverse lights aren't working. the Tail light was easy, I must have backed in too deep and cracked the housing, which cut a wire on it. I replaced the housings, hooked everything back up as it was, and I still don't have reverse lights. I checked the harness, replaced a bunch of worn and falling apart butt connectors with solder so I know the connections are solid from the previous owner all the way back to what is obviously stock harness. I believe all of the shotty work has been removed and cleared out.

My tail lights are now better than ever before. I am still however not got any reverse lights. the circuit they look to be attached to is putting out .032V with the key on and the truck parked in reverse, and the bulbs are brand new and tested good. I assume it is possible that the switch has failed, but getting at it is the trick. is it accessible without taking the cab off? is that circuit accessible from in the cabin that I could use a DMM and test it to see that it is/isn't working. is is possible to borrow that signal from another circuit and i could make my own Relay setup to click them on. This old girl has a lot of miles on her, but before I rig up a switch and turn the lights on myself when going into reverse, I was hoping someone could point me in the right direction.

PS This is my first post here Thanks for the add, found this forum googling my issue.

 

The reverse light shares a common ground with the brake lights, and is bolted on(I bought cheap generic trailer lights at the auto parts store as they are simple and get the job done). we also wire brushed to bare metal before bolting it down. I'm very confident that the ground is good, I've worked on enough old cars to know how much of a pain that can be. I have also used the brake light lead and swapped that to the reverse pin and the bulb illuminates properly.

I looked at the fuse box, but didn't see any fuses listed as reverse or anything close. is it on another circuit?

I figured the switch was pretty simple, this isn't a modern truck with all kinds of sensors on it anyways, and most everything is serviceable very simply. Just got to get down there and try and figure out which one it is.