If they are not too bad, use Meguier´s 2-part plastic polish. It is intended for plastic rear windows used on convertible tops, but it works on any plastic, taillights included. It takes a bit of patience and rubbing, but the results are quite satisfying. If, however, they are not coming around using that method, you can wetsand the lenses with 600 wet/dry (or smoother) before you use the polish. Either way, you will have to repeat the process periodically, since the lexan lenses will continue oxidizing for the rest of their lives.
Marcus