You can do one of the ssr polishes or optimum polishes. As long as the lenses you're trying to clean up are plastic/polycarbonate, and not glass, they'll usually clean up pretty well. You can do so on an Yellow (better for headlights and not tails, as taillights are usually softer) or orange pad (or white if you prefer a lighter cut at first). I've done some lenses with the cordless drill bit attachment. Put the 4" pad on the attachment, then apply a small amount of product to the pad and work it in to the lense.
If you have a dual action/random orbital machine (or other), you can use that just as effectively.
Heres some links to check out.
Attachment and pads:
Buffers, Pads & Accessories
Polishes, Swirl Removers, Paint Cleaners & Compounds
You can top it off w/ a sealant to help w/ UV protection too. I'd go optiseal. Its very versatile... use it on lenses, wheels, paint... truly great product.