This works Great:
Back-to-Black®
As your ride’s exterior ages, rubber, plastic and vinyl exterior hardware of all colors will dry, crack, flake and deteriorate. This oxidation can be stopped and reversed with Mothers® Back-to-Black®, our favorite auto-care concoction. This trim treatment is great for all sorts of exterior hardware in all colors — bumpers, molding, louvers, window trim, vents, seals, skirts, door handles, wiper arms-anything that’s plastic, rubber or vinyl. Apply Back-to-Black® to sun-burnt plastic, rubber and vinyl to revive the factory-new sheen, then re-treat that exterior hardware regularly.
Haven't used this, but as soon as I get a Mother's Power Ball I am going to give it a try:
PowerPlasticâ„¢
Mothers® PowerPlastic™ is specifically formulated for use with our PowerBall® line of tools to allow you to quickly and easily polish and protect a wide variety of hard or flexible clear plastics in one simple step. It features a special blend of oxidation inhibiting polymers, UV shields and an ultra-fine polish that work in unison to clean and clarify your headlights, taillights, convertible top windows and more.