You can clean them if the material is not damaged but you cant' clean away the discoloration from the exposure. You may have to clean then, prep them and spray a light coat of coloring to return them it at least a consistence look assuming that your not going to use the car allot since you can wear through the coating over time
I've used SEMS products in the past. If I recall correctly they sell an adhesion promoter also to help
Going to move this to the interior discussion area of the site