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When my headlight switch failed from oversize bulbs neither high or low worked. There's some sorta bi metallic thermal switch in it that overheats cuts out and when it cools comes back on and gets progressively worse.
What year truck? When the headlight switch goes out, they all quit. You need to get a test light out, and see if you have power to the ballasts when they're on. If not, see if you're losing power or ground. Did you install relays, did you add in dedicated harness's or use the stock ones? If you kept the stock wiring, you have a fire waiting to happen with that tiny ground wire that GM used for the headlights. GM used about an 18 gauge wire for both high and low beams to each headlight assembly, and to do the all on mod requires an add in harness, and relays. Otherwise you are begging for an electrical meltdown with those 20 gauge power feed wires and under sized grounds.