By the "bulb", I guess you mean the projector lens? If it's dirty, there is likely to be a hole in the light unit that is letting dust as well as moisture in. Perhaps one of the rear covers isn't secure, or a drain tube missing? The only way to check this is to remove the lamp. To clean it, you'll need to dismantle the lamp to remove the xenon unit. It is doable with care, but you may prefer to take it to the dealers and try them for a replacement light unit.