When talking about ABS or Polyethylene parts a soak in warm caustic soda soloution works well to clean them. To bring back the original lustre after it becomes dull or discolored from age/UV degredation, etc. there's is no way to do that, once the plasticizers in the material have changed or degraded it's not reversable. An application of topical dressing is the answer, and it will likely have to be reapplied from time to time to keep it that way depending on the environment the car is in.