As long as the red is still good, the best thing to do is to clear coat it. Glass bead it first so the clear can get a good gite. I've done this to many, many taillite lenses with excellent results.

It also works with many of the new headlights that get so cloudy. Sand them down good with 500 or so till smooth and use the left-over clear from a paint job to bring them back to life.