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Reverse lights don't work?

Ok so the GMC has another one of those tiny issues that we all hate to have. I don’t know if its been this way since I bought it or if its just since we pulled the transmission to put on a new clutch, but my reverse lights don’t work. every other light on the truck works and I checked the bulbs and the fuses and found nothing. I even checked to see if the sensor wire was hooked up to the transmission since we pulled it out and it is. I have an online subscription to Chilton’s and have looked at the wiring diagrams time and time again and can’t figure this one out. my real question at this point is how the switch in the transmission works, can I just rewire a bit and direct wire through it or does it have to go through the main harness somehow. the details on the truck are as follows: 1994 GMC K1500 5 speed 350 TBI. please help I’ve run out of ideas.

First thing is to verify the switch is working, or not. Do you have a manual xmission? Ok, I see you do since you have a clutch. probably the same either manual or auto, but I’m looking at the schematic now for the manual. 1994 GMC Truck K 1500 V8-350 5.7L TBI . It’s just a simple two terminal switch. There’s a dark blue wire going to it from the fuse block which should be hot in run or start. And a light green wire which goes to the back-up lights. The switch is closed when the xmission is in reverse. So ask your mechanic to test to see if the dark blue wire is getting power in start and run, and when in reverse the switch should turn on and the light green should get the power too.

Do you have a multimeter? Completely invaluable when dealing with electrical issues. First, use the ohmmeter to check the reverse sensor switch on the transmission. It should be a closed circuit only when it is in reverse. Then check the wire for continuity to the sensor. According to the wiring diagram I found, there should be two wires at the reverse sensor switch, a blue wire and a light green wire. The blue wire gets power from the turn signal fuse and should have 12V any time the ignition is turned on. The lt green wire sends that power to the back-up lights. Should be easy to check.