If it is small spots of sap and not large globs then you could try an automotive bug and tar remover or clay bar.
I had spots of tree sap on a new car which had sat in a lot for several months before I purchased it. The clay bar took care of it.
Don't take my advice. Go ask in an Audi, Corvette or some other auto forum. Those guys are obsessive about removing contaminants for automotive parts.
-Tim-