This thread has been made a sticky because of the ongoing fogging issue.
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Additional info - Jan 2009
The fogging of the headlamp units is something we are destined to live with. I've now replaced the lamps on my blue car once, and on my silver car twice due to continued fogging.
The Z8 Club are aware of this issue, and have told me that it was even noticed during the time of original production. At this time there is no known cause or cure. I hardly ever have my lights on, I live in the desert climate, so there is no inherent moisture, and my cars live in a heated garage, both have very low miles, and are always gently hand washed, so even in the most ideal of conditions the headlamps fog up.
Here is a PDF on changing the lamp units should you decide to do it yourself. It is around 20mb so may take a moment to download.
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Text of original post
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. As yet no one has taken the initative, or had the desire to find out if they can be cleaned.
I don't understand why the units are sealed, it is my understanding that this has nothing to do with the bulbs, reflectors or lenses which are the key components to projecting light. I think it has more to do with inerrt gas in a very hot/cold environment, but I'm not sure. Once someone figures out the what, why and how of the sealing we can design a work around, then 'open the egg' to clean the lenses.[/i]