Tried it, doesn't work

 

you should get your self some small desicant bags and put them in the housing you should only need one or two.
my lads headlight did the same thing after he fitted an hid upgrade but the bags cured the problem very nicely.

take a look on fle bay

RS components do them too.

 

They dont do it on a new car.
So somethings broken.

My money is on the seal behind the light cluster -pull it and repalce the fibre gasket on the flitch panel.

This also causes water in the boot too

 

Was just lurking on this forum looking for pictures of European S2k's when I noticed this post. My S2k just had this problem surface in one rear tail light after a trip in the rain. I removed the tail light, baked it in my oven and then used a black gasket sealer along the seam entirely around all edges of the tail light, as well as all of the small clip locations which can be sources of leakage. It was pretty easy to do and so far it has held up fine, hasn't re-occurred on a recent rain trip. I caught my leak quickly so there wasn't much scale deposits/stains inside the light.

I first baked it at 170 degrees F but the moisture wasn't evaporating, then moved the temp up to 225 degrees and as soon as I did that the water disappeared. Hot enough to evaporate the water but not hot enough to damage the light. Had it in there for about 5 minutes once the proper temp was reached. I placed the light fixture on a wood cutting board rather than on the metal oven racks directly.

My lights are in mint condition so I thought it was worth trying. So far so good. I think the trick is using a proper sealant and getting it along the entire seam that goes around the entire light fixture on all sides, plus getting it into the small clip hole locations as well which are spread along all surfaces of the fixture. hth.