The one way I found that worked for me is take the bulb out and let it sit in the sun all day. If you don't have that option is to put a heat lamp or heater pointed at the headlight for an hour or two to dry it out. Once it's dry put the bulb back in.
Now you should probably find out why it's fogging up. usually means there's a leak or gasket missing. need to seal it up.
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