Well, you can see my lights were not TOO bad- I try to stay on top of it. The left one (in the picture- car right) was actually worse than the left one I have not cleaned yet. It was/is impressive!
The stuff is from Pep Boys (probably other places have it) called 'blue magic' brand cleaner for headlights and they also have a sealer to use after cleaning. I gave it a shot after the success with the Blue Magic upholstery shampoo...
Procedure (how I did it)
:
0) pop the hood and mask around the light...
1) clean three times with the invisible glass cleaner (or whatever you prefer)
2) clean three times with the blue stuff. I put it on liberally as it has ammonia in it and actually cleans as well as 'abrades' the surface clean. feels like a rubbing compound... I used an old shop rag (coarse cotton) to apply it and rub it off. You can see the yellow/brown on the rag (looks like tar from a smoker's dirty window!) BLEEEEAAAC! GROSS!
3) clean three times with the invisible glass to get the white residue off that you can't buff off (it gets harder to buff as it dries).
4) If it is not clear enough for you, repeat step 2. Or, as they suggest, use 2000 grit wet to get the grunge out and
then clean with the blue stuff.
5) apply the sealer. This is a slightly oily clear fluid that you spray on and wipe off. I sprayed it on (quite a bit) and worked it in well with my hand, let it sit a couple minutes and then buff it off with an old towel.
The sealer is supposed to keep it from yellowing and make it easier to clean next time. I coated the lights on my company car (2011 Taurus) so hopefully those will stay clear...
No more ti. 
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