1998 Chevy c/k 2500 rear light issue

This will go full screen, if it's any help. I am thinking ground somewhere also I don't see any wiring to third brake light, and according to my source they were required beginning in 1986 for cars and 1994 for light trucks. Does it have one? Maybe something to do with that..? Throwing guesses

I thought it might be a ground issue as well...but if it were then the right hand circuit /light wouldn't work either but it works fine when hooked up to either light. Thanks for the insight .... it is very perplexing

Do you have a trailer hookup? I have heard issues with that hookup causing problems with the tail light system If you can unhook it easily see if that works, Sometime the pins short out or become corroded causing issues. The wiring diagram looks like it comes off one fuse. If you have one set of lights working, then you have power past the fuse. Check where the wire splits off to go to the set of lights not working and follow it to the bulbs. If you have power to the bulbs but not through the bulbs, you may have a ground issue. Check, starting at the common ground, clean the contact connection and make sure the wire is not cracked or "open". Put a wire from the ground side of the lights to a secondary ground and see if that works. If you have power all the way back to where the light wire splits, then that is where you start. If you have two different wires running back and one fuse, start at the fuse box and go backwards toward the lights. You might even try changing the bulbs and make sure they work. Stranger things have happened. If you don't understand the wiring diagram, ask around for someone to help read it. It will save a lot of time and answer a lot of questions, plus give you a starting point.