it has been my experience that the contacts will get funky and sometimes even melt the plastic a little from the heat, I would take a good look at the contacts and be sure the bulb(s) are good, sometimes part of the bulb will burn out if it is a 2 filament, if the contacts have any black buildup (residue) try cleaning them first and bending them in a little so they have a good firm contact, my license plate bulb was flickering recently and I just cleaned it up and tweaked the contacts a hair and works perfect again. but I have had to replace the end harness before because it was just shot, on all these gm's there is end harness under the rear bumper (directly under the left/right sides) that feeds up to the lights it can be replaced by just unplugging it and plugging in a new one if need be.