Maybe a useful method to clear up those ugly yellow lenses. Laquer thinner on a rag used to remove paint overspray, also removed the yellowed surface of the headlight lens. Seems to soften the degraded plastic and disolve it. Some care with a clean cloth can leave the lens as clear as new. Apparently the yellow is on the outside. I gave three lenses this treatment with good success. Wet & dry sandpaper, with successively finer grits (400 - 1200) also works well. Polishing compound to finish like new.