I used Meguiar's Ultimate Compound on my daily driver's headlights, which were VERY faded and yellowed, and they restored to this condition:
Usually, just a slight fading or yellowing of the lights can be cured with just a polishing compound. However, be warned - whatever method you use, you will be stripping the rest of the UV protectant off of the housing. You will need to seal the lights again using a UV sealant kit or equivalent.