[MWCC]

These GM trucks were notorious for problems like you're describing. Most of the time the fault is the injection molded socket strip that holds all the bulbs you are having trouble with. Check to see if the pins are corroded or broken off where the harness plugs into the strip. Also look for any cracks in the strip itself. Chances are one or both of the conditions I described are the cause. You can get a new strip from a good auto parts supplier or the dealer.