There is a chemical sold in most auto stores. I forget the name of it, probably something like Tint-off. It's a blue chemical in a spray bottle and it is designed to cause a chemical reaction with the glue which breaks it down to the point it can be peeled away with a credit card. Don't use a razor over the defrost coils.

I had to use this to de-tint an accord I bought that had a DIY tint job on it that was bubbling. The spray saved my life damn near, it would have taken me forever otherwise.