If it's a surface film you could try one of those magic erasers with some mild soap. Or you could try that kit they sell to fix yellowed car headlights (which is probably the same thing!). If the sun has caused a chemical change in the plastic, replacement of the 'glass' may be the only option. Do you know if you can remove the 'glass' from the frame?
BTW, I have not looked real close, but I assumed my portlights were glass, not plastic...but I could be wrong.