Been having this problem awhile with my '98 GMC Jimmy 4x4. The low beams do not turn on at all and recently the driver side high beam went out. All running lights work fine. I have replaced all bulbs with no result and checked for blown fuses but found nothing out of the ordinary. There are two 5 prong relays in the fusebox labeled headlamp ground and headLamp power and have interchanged the two and nothing happened. I poked at the harness with a circuit tester and the driver high beam is receiving no power but the low beams lit up on the tester as red (ground? I believe) any thoughts or solutions would be appreciated.

 

High beams, low beams, dimmer switch, flash to pass, daytime running lights, and auto lamp circuits all use a series of relays, a diode, (98 only) positive side switching and isolated ground side switching. Several of the circuits go through the multifunction switch, some of them go to the BCM, and the circuitry gets quite complex to say the least. If you have a soild understanding of DC circuits, a digital volt ohm meter, (and are proficient with it), and have lots of patience and time, I can walk you through the diagnosis. I helped one of the members several years ago, with a very similar problem on a 2000 Jimmy, which is very much like yours. Ended up being a broken, (burned) bus bar in the underhood fuse panel.