2000 Excursion without for lights. Sunday morning woke up with no low beams, and no lights on the running boards. High beams work, all brake lights work, and flashers work. Daytime running lights work, but as soon as I turn the headlight switch all the way on, the headlights go dark. Both headlights work in the high beam position. All fuses checked out okay, but just to be safe I replaced any fuse related to lighting on the excursion. Both bulbs seemed okay, but I replaced both with good spares. In the process, I noticed the passenger headlight socket was a little corroded and melted. I figured now was the perfect time to install my 05-07 headlights with new H13 pigtails (spliced in, no adapters), and the problem would be fixed. Install went smooth, however the problem still persists. I still have no low beams, and no running board lights. I removed the headlight switch, checked for burnt pins or chaffed wires, all seemed good. Did the same to the multifunction switch, and took it apart and cleaned it, all seemed good there as well. I'm fairly lost now and don't want to just throw new parts at it without knowing what's truly wrong.
Any ideas would be greatly appreciated.

 

I think I'd start by (or go to) testing the line(s) for voltage. Check that there's power on both sides of the fuse box and switch, then check that power's getting to the headlight assembly.

That'll help isolate the problem.

 

I found and fixed the problem. One of the DRL relays located behind the glove box was bad. Replaced the relay, and all is good again.