This is a recurring theme and there is plenty of discussion on the subject on social media. When we bought ours the left one was foggy. The dealer agreed to ship me a new one and I replaced it myself. Upon examination of the old unit there really was nothing obviously “wrong” with it. In the meantime the right one had also started fogging up.
There are different degrees of water intrusion. Some report an inch or more of water standing in the unit. That’s obviously a serious leak and needs further investigation. Most report condensation/fogging.
My personal theory is that the unit, which is not air tight and filled with dry air like one would wish, has SOME exchange of air with the outside, but not enough to keep it properly ventilated. So, at times humid air gets in there, then the temperature drops and the moisture in the humid air condensates on the lens. Over time it WILL clear, but not in the next half hour, or three days, unless maybe it ends up perfectly sitting in bright sun. Good news is that when you turn the lights on the immediate “glass” in front of the bulb clears enough for the light to be functional.
Some folks have removed the unit and sealed the you know what out of it with silicone and report good results.
My solution is the following: If you slide out the generator and look on the backside of the light there are a number of “tubes” in the molded plastic that lead into the unit. There are little plastic caps on them. I use a hair dryer, some hose and duct tape to force warm air into the unit until it clears. Takes 15 minutes or so per side.
So, some claim a fix, I say there isn’t and that it’s a matter of treating it when it happens. One COULD get into rebuilding/replacing the units with something different and better. This could also take care of the marginal amount of light these things put out, but that’s a different story for a different mod.
Since I dried mine out last fall (when temperatures first dropped) they’ve been clear all winter.
A year and 13,000 miles later the lights are still clear. Go figure. Now not sure what conditions caused them to fog up in the first place………