I have used this stuff for years - enclosed plastic headlights, instrument gauges, motorcycle windscreens, helmet faceshields,. It was originally developed for aircraft lexan windows. Available at most auto parts/accessories stores. Very easy to use.
I polished my lens the old fasioned way. As my car is an '02 which has been driven at high speeds rather regularly, my lenses had their share of sand blasting and pock marks. No polish will remove that. Thus, I wet sanded, then polished using progressively less aggressive polishes, to get the outer surface like glass. Worked fine. Problem is that pesky film INSIDE the lens which makes the whole thing look cloudy, despite all that work.
I removed my front driver's side wheel, then took the front fender access cover off, and went so far as removed the back of the bulb housing (it's a screw cap of sorts). I saw no realistic way to get any type of brush or cleaner into the light housing. In fact, whatever I did caused a HID warning light to come on whenever I turn my lights on. I must have inadvertently disconnected something, as it's pretty tight in there.
Incidentally, agressively polishing the lens as I did will require periodic application of a UV coating, which is readily available. The sanding and/or polishing can remove the "permanent" UV coat applied at the factory.