Headlights Out '90 K1500

Yesterday I jumped my Grand Cherokee with the trusty plow truck. Tonight (first time I've driven the truck in the dark) the GMC's headlights don't work. Everything else seems to be working fine. I can't find a fuse to blame. Any ideas why the headlamps are out and, more importantly, how to fix 'em?

Thanks,
Repairman

Pulled an overnighter with nice wiring diagrams found at www.autozone.com. Tracked 12V from battery to high/low switch input and both high and low outputs... Only sacrificed one knuckle in the process. When I proved the 12V on the output, the lights came on. Dim only - this is where I started. Now to figure out why the brights don't work...