From what I can remember, the W140 S500 we have at home has glass head lights, not plastic. Are you SURE they are plastic? The indicators are plastic, but the head lights I am quite sure are glass. If I am wrong, I apologize in advance (I've never paid too much attention to this, it was only recently that I discovered the head lights of my E420 are glass, and I just double-checked them with the W140...they make the same "clang" when I tap 'em with my finger...so I assume it is glass too). When you say they get "cloudy", are you sayin' they fog up, or there is some sorta cloudy residual to the glass? Have you attempted to wipe the INSIDE of the glass with Windex or something...and then make sure they are super dry and clean before putting the lense back on to prevent condensation? I dunno...just a suggestion...never had the problem before...
1995 black pearl/black Mercedes-Benz E420