Most of the time the fog is due to the vent tubes missing. They can fall off or even get knocked off while cleaning the engine compartment. With the 90 degree tubes missing, water runs down the inner lip of the top of the fender, directly into the light housings.
James
Originally Posted by onehundred80
Severe fogging in the lens is due to leakage of water through the seam. This can be fixed, after drying out the lens, with silicone caulking. I think that DTRL's prevent the minor misting due to humid conditions.