Any body know or done it, How to clean inside of the Headlamp Housing?
or
How to restore the yollow Headlamp Housing back to crystal clear?

You'll never get yellow ones to look new again- if it has to look perfect, you'll have to spend $$$. But you can get rid of the yellow, using spray oven cleaner. This is best OFF the car, like in the sink, but you can do it in the car if you spray the oven cleaner on a rag or otherwise avoid getting it on any paint. Oven cleaner is harmless to plastic, but removes the yellow coatign that is basically air pollution attacking the surface of the cleaer plastic. Afterwards, the lens will be cleaner but may have a dull finish- a plastic polish or wax applied then will bring it most of the way back to life. If they're both faded, it'll look much better- if one's new, and the other's yellow, you'll still see the difference but it won't be YELLOW like it is now.
Good luck!
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total

look down a couple of posts to one called "Question about headlight assemblies... " and if you do a search you'll find another post about using polishing compound to get rid of the yellow... good luck.