Try some Meguiar's PlastX Clear Plastic Cleaner and Polish. If that doesn't work, you can step up to a slightly more aggressive polish and continue up from there.
try megs 205. If that doesn't do it, then 105, then 205 again. Polish after the 205. Depending what products you have at your disposal, if you don't have 105/205, Megs UC might do the trick...or PlastX as another member mentioned.