I accidentally deleted two threads I was trying to merge on the subject, so here is what we know about this problem so far.
The lens units are meant to be sealed, but they let in moisture, and many owners have reported fogging, or a filming over of the inside of the lenses. As of yet there is no solution that we have in our combined knowledge base to deal with this issue, and BMW simply say we have to replace the units.
Since this is both expensive and unnecessary I am hoping that someone will come up with an efficient and easy fix to the issue. I would think being able to clean out the units with some kind of lighter fluid or lens cleaner would work, but I have no idea how one would go about doing it.