if you got a white one go ahead n use the smallest bit of bleach and a toothbrush(dont worry your plastic will be fine, already had to use it)...
or mud might be able to do it. I went riding at my buddys gpas farm, cow shit everywhere. got home didnt have time to clean her off, 4 days later when I did the cow shit had obviously stained onto my plastic, wouldnt come off with anything! not even the bleach...obviously had no choice but to say F it, went ridin again a week later, (dif place, no cow shit), real muddy ridin for about 5 hours, hosed her off right after, and as God as my witness the cow shit stains were gone after I hosed off the mud!
So you could also try going muddin!