I have never had this problem and don't have a real sollution, maybe someone has some ideas. Sorry.
You could try(again on a small area) wiping the plastic with some lacquer thinner, it may soften it enough to peel. You will probably have to repaint it after this if it works.
My only other idea would be using a putty knife and scraping it being carefull not to scratch the metal to much, depending on the damage to the paint or primer you will probably have to sand, re-primer and paint.
Another thought: since plastic is petroleum-based, maybe some light weight lubricating oil would get it to "release" it's hold on the metal. Try to get some started between the plastic and the metal.
[This message has been edited by rpxlpx (edited September 05, 2000).]