First take it off, dry the inside out with a blow dryer. Check and see if ther are any leaks aroud the outside of the housing, if so use some silicone(I would do it anyway-can't hurt). If not, condensation may be getting in from the bulb housing. Then get some insulation tape(home depot-AP/Armaflex from Carmacell, self adherhing-4 or 5 bucks) cut a 2x2 piece, cut a hole in the middle and make youself a washer so to speak. Should do the trick.