Often the electrical design of the vehicle would feed the headlight switch from the fuse box. The headlight being a heavy consumer would have its own separate fuse. Further being a heavy consumer denotes high current passing through the wires. It is also possible that the connectors (at different points) would have been burned or corroded. The best order of checking would be fuse, fuse snaps/cradle (where the fuses are inserted), fuse block and/or fuse block connector(s) including the internal jumpers and wire/wiring harness. A DVM would facilitate checking the +12 at various points.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.
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