Well, I have the SAME EXACT CORNERS on my car, and yes, I got them from eBay, and yes, I think they look MUCH better than the "frosted" ones. You can all tear me up later.... on to the question...
Two things... as Rooz had indicated earlier, silicone them all the way around. I did this as well. It works.
Also, you'll notice that on the inside of the lens, near the crome part, you'll see there's actually a hole. It's covered with a sticker-like thing. Take it off. This is (to my knowledge) a vent hole that lets air in and out. It's covered while they're new so as to not let dust/dirt get to the inside of the lens during storage/shipping.
Check out my site. I have the same lenses (absolutely no moisture) and I think they look fantastic.
"In the road of life there are Bimmers and there are Bimmers"
Mods: Full JDM lighting, APC bulbs, ///M rims, Four wheel Brembo rotors, Bilstein Touring suspension.