I posted this a month or two ago in the GMC forum and never received a response so I am posting it here in hopes that someone can help. Thanks.

Hello. New to the site as of today and new to Sierra ownership, today as well!!

Ok so I purchased a 98 sierra w/a 350 as a second vehicle and regular workhorse.
It was an old fleet truck and has a ton of miles on it. I drove it this evening only to find a dilemma already in my purchase.
The breaklights and the cargo light work only with the light switch in the off position. Hazards work at all times. the rest of time, light switch on or in reverse or even the tailights that should be on at all times, they dont work. Now I said it was a fleet truck and it was sold to me sans rear bumper so any number of things could have been done to the original wiring harness. Im now slouch in the garage but I do have a quick temper and wrenches have wound up through drywall.... and into the living room making the mrs very happy! I was wondering if there are any tips, tricks, diagrams or, unlikely in this sit., any common issues people have seen before to arm me before battle.

My drywall thanks you.

 

I think its the switch. I had the same problem with a different vehicle but the switch was really hard to find and very expensive. I ended up finding out which wire was which and buying a 2 way switch from auto zone. It was kind of cool because I could turn my lights tail lights off but leave my headlights on. The bad thing was I would forget sometimes and was reminded buy the police. I never got a ticket but I did have to explain why it was set up like that.

 

I agree it could be the switch, but the rear bumper missing tells me something happened back there that could be the culpit, especially with taillights and brakelights not working correctly. Do the backup lights work correctly?