sounds like you have a ground issue on the front lights. if you have a couple of jumper wires, like a test wire with alligator clips, try using them from a good ground to the light housings and see if the problem goes away. I have a cheap old light duty set of booster cables that I found in a wreck, with cheap clamps that don't have much squeeze pressure, and they have come in handy for such things. they have a larger clamp so they stay attached to things like light sockets but they don't squeeze so hard they crush the light socket. if you don't have any test leads that will work just use some mechanics wire, fence wire or whatever you have laying around, it doesn't need to be insulated as it is a ground wire so unless it touches something that is battery positive you'll be fine. vice grip it to the frame, run it under the truck where there is no battery positive to short out and then touch that to the light sockets with the signals on. maybe you find your problem.
since the GMC mustache signal light housings are screwed to the mustache, and the mustache is screwed to the fenders and other grille parts, and they are also screwed to the rad support, etc, there are many spots that there could be a bad ground. that is why I suggest to run a dedicated ground wire to the light and eliminate all the other possible bad connections. like LG says, solder a wire to the light socket and then ground that wire to a good clean spot on the frame, with a star washer, and when you are certain it is all good coat the connections with some grease or at least shoot some spray bomb paint over it. done.
https://www.grote.com/electrical-con...alant/99170-1/