It's possible that it's fogging up because there is a small crack or hole in the housing, causing condesation to build up. Or it may not be sealed up very well. Has it been fogged up for a long time? If so, it wouldn't hurt to get used headlight and replace it. I had a headlight fog up once and it went away within a day, so if yours has been like that for awhile it might stay that way. I would look around salvage yards or ebay for a replacement.