Are your plastic headlight lenses fogged or turning that nasty yellowish-brown color? Try this, it works great!!!
Using 180 grit sandpaper, by hand, lightly sand the entire surface until it becomes bright white, looking similar to frosted glass. Clean away any sanding dust and then spray the sanded areas with a quality gloss clear coat. I used Rustoleum in a spray can and my lights look and work as if they were new!
Try this, you will be absolutely amazed!