My Lexus SC400 used to suffer from intermittent headlight lense fogging. It used to drive me nuts. Then on the Lexus board, I found out that there was a elbow shaped rubber vent tube that allowed air to enter and escape from the lamp fixture.

It appears that some industrious spiders would take up residence inside the vent tube(s) and clog them. Thus they stopped working and the moist air would have a hard time escaping. A swab with a Q tip dipped in RAID ant poison was the solution. Perhaps there is something like this going on??