If 409 and Clorox doesn't do the trick try moving up to a scotch bright in a corner where it's not noticable from the back and see if it lightens it. You are basicly sanding it so be easy about it. If it doesn't lighten then the lens has discolored through the material and the only fix is replacement. www.inlandrv.com
is probably you best bet. Andy has a lot of older stock.
If it does lighten up then find a plastic polish (Ace and auto stores would be a good bet) and have at it.