I have had very good luck drilling 2 holes in the rear plastic housing. One hole is drilled at the lowest possible point for drainage and the second hole at the highest possible point to allow vapor to escape when the bulb is on and hot. Although it is too difficult to confirm it appears to me that moisture is drawn in at poor sealing of the mating surface of the lens and rear housing due to the effects of drafting. The price of some headlight assemblies is more than I paid for the first car I owned.