There isn't water getting in unless they are broken, just with moist air trapped inside with the back sealed up this condenses into water which will sit down in the lowest parts of the headlight. As they cool at night this then causes condensation and a ever going circle, lights on Clears, lights off, cools and mists up again and again.


It can only need a thimble of water to cause condensation.
The only real way to fully clear the issue is to leave the backs off, and leave the car parked 3-4 dry days somewhere facing maximum sun light. This way all the water will evaporate. Even from the bottom of the lens case.
You can always add a few bags of Silica Gel (strung together on a thread) and in 4 days run the car, turn the lights on for a while so they warm the air inside, and pull the thread out. Then reseal the rear of the headlight casing (if yours seals are really bad old and decayed apply some non-setting silicon around them before sealing, and Bobs your uncle.


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