I've done some research in the past on these "Plastic headlights" and found that, indeed you can clear them right up again, the only problem is you eventually remove the clear coat/protective clear that's applied to them and they go bad really quick, in the sunlight, without this barrier. I've tried to track down what they use for sealing these when manufacturing the assemblies, but I've come to a dead end, as it's been reveled as a "trade secret". If anyone's out there who does bodywork for a living, he/she may have an idea of what they use. I had a problem in the past, with driving lights becoming pitted from road debris, to the point of having a sandblasted look. I cleaned them up to factory condition by water sanding them with P1000,P1500,P2000 water paper, but in doing so, removes the clear that's applied from the factory. They lasted great, but I did have to make sure I had a UV wax on them. Seemed to work.
Those aftermarket HID kits should be banned! Nobody can see anything with those coming at you or from behind you. Just a big scattering of lights. The worst car on the road today for tail lights is anything VW. They are dull and small and can hardly be seen at night. I don't know how they are legal. They are worse than those people with the black or smoke tint over their tail lights.