They might be oxidized like plastic car headlights get after they're in the sun for a few years. When the lenses get oxidized they only get yellowed on the surface, and there are kits at most auto parts stores to remove it. I never tried the kits myself so I can't say for sure that they work, but it might be worth a try. I think the kits come with some buffing wheels for a drill and some kind of polish and wax.