GMC Troubleshooting Questions? Ask a Mechanic for Answers
Have you replaced any parts?
Do you have access to a test light or voltmeter?
All bulbs for high and low beam get voltage from the same source, so there should be voltage there if high beams work. Try grounding the Yellow wire at either of the low beam sockets and see if the low beam lights come on (make sure headlights are turned on). Let me know what it does.