First you should try to locate how the moisture is getting in. Being that you had just replaced the bulbs, i would look there first. Do the bulbs fit nice and snug into the housing? Are there any "gaps" around the bulb? If they do not fit snug, you will have to try and snug it up or fill in the gaps, maybe with some kind of compressible foam to act as a sort of gasket. If you determined the bulb fitment is not the problem, start looking for cracks/separations in the plastic housing. If you locate cracks in the "clear" part of the housing you can seal with aquarium sealer (which is clear). If you locate a crack on the black portion of the casing, just use some RTV and seal it up. Hopefully this is enough to get you started. I would definitely start with the bulb fitment since you stated you had just installed the HIDs.