I dont think anything that has been suggested will last further than the next shower of rain - if the plastic has been damaged ( usually by sunlight ) then nothing will repair it - only gloss over it temporarily.
But as Toureman has suggested, I would also use back to black or Armourall tyre foam is another good one to try.
hi, not all cars are the same plastic,ive found this out. i use wd40 auto glms bumper gel and once its back to nearly black i wipe it over with either shoe shine sponge or baby wipes!! honestly. i used to enter my car into concourse comps and it did the trick!!