My 98 Sierra has no brake or reverse lights, nor does it have rear blinkers. Its not a fuse or bulbs, and they quit working at different times. Any suggestions? Im gonna try the brake light switch

had the same problem with an 03, look at the back of the truck between the frame rails and see if there is a module, check the plugs on the module, the 03 one plug was loose. if no module, and all lights don't work, then trace wiring, you have a ground problem some where.

Also try tightening the check valve on the brake booster.
I have NOOO idea why but that solved the same problem on my 89.