Is it the headlight itself, or the housing around it? ("headlight door" they call it, for some unknown reason...)
If it's the rectangular headlight itself, just replace the thing.
If it's the 'door', it shouldn't get wet at all, and is sealed fairly well, so you'll likely need to remove it, and either split it open to thoroughly dry it before resealing it, or drill a small hole in one corner and brace it so it can drain out that way.
If you drill it, I'd suggest finding a way to get air flow through it, so it doesn't just condense all over inside.
Neither of these is probably 'proper' but I suspect that would involve an expensive trip to the dealer for a new part.
Anyone else have any ideas?