Here's what I managed on this topic (picture taken at night under a regular (yellow) light w/no flash). The lamps both came off the same car.
Lamps turn yellow after the factory coating wears off.
These were so yellow, none of the Meguiars/3M/toothpaste methods worked (at least by hand). So... out comes the sandpaper. I had to drop back to 320 grit on the palm sander to clear the yellow, then 600 and 2000 wet sanding by hand. The results were pretty good, but not factory clear.
Under a strong magnifying glass, the surface was still rough. Here again, none of the ScratchX/PlastX/whatever would get it any smoother. I don't know if a buffer would have done it, because I used "Blue Coral One Step Touchless Acrylic Instant Restoration Coating". A few careful coats of this stuff look pretty good... for a year or so.