A Product called Micro-mesh if it's really bad otherwise some plastic polish should suffice! Here is one kit ,but do some research there are different kits,this is just an example...
Micro-Mesh Finishing Kit
This is how i clean plastic.
1) remove dirt/dust with swiffer or microfiber cloth,or dampened eyeglass cleaner cloth
*Note USING VERY LIGHT PRESSURE
2) Use plastic Cleaner Polish Use Sparingly (apply to microfiber cloth )
3) Using New Clean Cloth Buff using Slightly firmer pressure
(Horizontal Side motion only lessens the appearance of swirls and lessens appearance of scratches)