Zaino Brothers makes a Plastic Cleaner/Polish that also works great.

 

Dishwashing detergent and a tooth brush works pretty well. I had to do this about two or three time but it does clean it up pretty good. After I got mine cleaned I used some Meguires protectant on it and now I always tape before waxing.

 

I have used Simple Green and a tooth brush and it worked well.

Laurence