I just washed my headlights' interiors. They had obvious buildup of fine dust and it had been annoying me to see them so obviously dirty. They have drain holes in the bottom and I was rinsing off my car at the time, so I stuck the hose nozzle (adjustable fine spray to stream) into them and rinsed them clean. They haven't dried yet, which sort of worries me, but I had the headlights on for about 20 minutes to heat them so they would dry.
Yes, this was a little rash, but I've seen mention of the washing before and the drain holes made it worth it. The water that I accumulated in the lights was noticeably brownish. Yuck.
I'll post back with any problems I encounter.

-Will
PS I live in CA --in a dusty cornfield, so my dust situation is somewhat unique.

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1990 740 Turbo, on its way to stock specs, maybe beyond