I have a few ideas for you. Clean them first with hot water and any type of car wash soap and scrub them good. Next, use some type of either tire dressing or a plastic restorer. Wipe this one pretty thick and clean off the residue with a clean, dry cloth. Next, polish and wax with your favorite car polish. I have found that the tire dressing along with the combination of the polish and wax has made the lower panels shinier, plus they don't get as dirty from the junk kicked off the tires.