DIAGRAM 1: With Quad Headlights
The above typical headlight circuit diagram applies to the 1996, 1997, 1998, and 1999 Chevrolet/GMC 1500, 2500, and 3500 pick-up with Quad headlights.
NOTE: If you have QUAD headlights, then you'll have a separate bulb for the LO and Hi beams. If you have sealed beam headlights, then you'll only have two headlight assemblies with each one have a LO beam and HI beam filament.
Testing the headlight switch isn't dificult. You can find the how-to tutorial here: How To Test The Headlight Switch (4.3L, 5.0L, 5.7L) (at: troubleshootmyvehicle.com).