Hello Sharpish,
i would start with the switch, remove it from the dash and carfully take it apart if you feel so inclined, and clean it with an eraser. if not order a new one and change it out. then start cleaning gounds in the headlite harnes. Your lites are pulling a ground from somewhere they should not be. Most likely place in behind the left side headlite where it all hooks to the bay harness.
Good Luck
and
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