The root of the problem is somehow there is moisture getting in the housing. You have to find that first or else any water you remove is just going to find its way back in. The easy way to get the water out is to remove the bulb so the moisture has somewhere to go and then heat the lens with a hair dryer. It would be easier if you could just shoot the heat right into the opening where the bulb goes, but the hot air has to have somewhere to come back out from or else the moisture won't leave, so stick to the slow way which actually works.
Then find the crack and seal it with silicone.