Could be a faulty ignition switch but sounds like you may have a bad voltage regulator. Had one on my sled that every light worked accept headlight. I swore it was a wiring harness problem. Long story short the regulator couldn't handle the juice of the bulb due to internal issues. New one solved it. Using a voltage meter you can hook onto the light plug and test your voltage as you shut it off. Numbers will be all over the place if it is bad.
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