First, check the bulbs. Then use a test light to make sure power is getting to the socket. Then, as Charlie said, check all the grounds (as in remove & clean them). GM has always been bad about a blown bulb or bad ground making weird feedback things happen.
HTH,
~ Paul
aka "Tha Driver"
"Political correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end." - R. J. Wiedemann LtCol. USMC Ret