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.