I put this in uppercase in the hope it will save someone a few pounds as it did for me. In recent floods I got damp in a headlamp unit and it misted up completely. I took off the rear rubber seal (the easy to get to circular one) and taped a bag of drying agent (probably calcium chloride) around it. That didn't work, nor did a (fresh!) bag of silica gel.

I found an old 12v computer fan that fitted exactly over the opening. I taped it in place so it blew inwards, connected it up and ran it for about 24 hours. Scared it would flatten my battery I actually used a small motorbike battery that I slipped down into the space behind the light.

The headlamp cleared completely. I removed the fan and replaced the rubber seal. That was three weeks ago and it hasn't misted up since.

I removed the unit and put it on a radiator.

Remove the bulb(s) play a hair dryer to get a through flow of air. Avoid over heating the plastic. Used to work on my old Passat.

I leave them inside the house overnight without the bulbs in , the rubber gromits are a waste of time, I have not had a problem since I removed them