First of all, occasional condensation is normal.
If it never seems to dry up, that's not right.
Some headlight manufacturers add vent holes or slots in the tops, bottoms or at the back of the light housing so that any condensation accumulation can evaporate and to allow for pressure differences within the light housing. There are a few ways that condensation can form inside your lights:
The most common cause for excessive moisture inside your headlights is in the seam where the lens meets the housing, the extreme heat generated by the halogen or Xenon bulbs will dry out the adhesive and cause it to develop tiny cracks that can allow water to access it.
Many after market headlight lenses can leak in this way when they are brand new, because they are sub-standard and may not seal well during manufacturing.
If water can come in contact with the back of your headlight housing, then you should make sure the seals & gaskets are good around where your light bulbs go in. this applies to all the lights on the vehicle
How it works:
If your lens is not leaking, then the cause of condensation buildup inside your lights can work much like when your windshield fogs up from driving through a warm moist air pocket while driving in cold weather. The temperature differences from inside to out side can cause fogging of the windshield, and this can happen with your headlight too, but usually doesn't last long.
How do I dry out my headlights?
The best way for your average Joe to dry out his headlight lens housing is to remove the light bulb and in some newer cars the larger rubber plug and use a hair dryer, but don't cover the entire hole, keep the dryer away from opening so that the moist air can escape, and do not set dryer on the hottest setting. For light to moderate condensation, they should be dry in a matter of minutes. The hair dryer works much like the defroster for your windshield.
Below is a video from Kent Bergsma a Mercedes mechanic, and he discusses some possible reasons why your headlight may be accumulating moisture inside, and he also demonstrates what he does to fix the problem on his own vehicle. This is a common problem, and the principles apply to most vehicles on the road today.
Note: The strapping tape that he talks about, is a clear tape that has nylon fibers running parallel to the linear part of the tape that prevents it from stretching.
I hope this info helps!
By Scott Summerson
Hello, Scott Summerson here, Here's a little about my experience: