Meguiar's Plastic Polish - originally made for polishing out scratches in convertible top windows, motocycle helmet face shields and fairing windshields. Can find in auto parts stores that cater to import and motorcycle owners and motorcycle shops. In a pinch, you can try toothpaste or softscrub. Simichrome Poli might work too as it's abrasive is very fine, but it's made for metal and may be too harsh or have petrochemicals in it that might degrade the plastic.
.bh.