You want Meguiar's Mirror Glaze Plastic Polish.

http://www.meguiars.com/product_showroom/showproducts_template.cfm?SrcLine=BI

You can find it at auto parts places, usually near the car wax. It's specifically made for cleaning and removing scratches from clear plastic, like instrument panels. I use it to remove the scratches from my motorcycle windscreens and helmet faceshields.