you might want to try the 'Tar and Bug' remover for cars that they sell in the car-wash and detailing dept. I've also found that Goop is great on a lot of things, put it on, let it sit a while, you might just surprise yourself.
You may have to do it in several stages, maybe even 'convince' it to come off with a non-marring scraper like a plastic putty knife.
Hope this helps,
M58