You could use some cellulose thinners,this can be bought at a automotive parts shop or decorator centre. Try a very small area first in a not so obvious place,dont leave it on to long as it may eat into the plastic etc,wash it off with some warm soapy water.Read the safety guide lines first on the bottle or tin. Best to do it outside and no smoking etc. I use it alot in my job as an effective paint remover on glass and metal. After you have finished try using a silicone bumper shine to restore the plastic back to its original finish, again obtainable from a automotive parts shop. Good luck
Rob
November 2008