The cheap tinting on the windows were bubbling. So I was able to peel the film from the rear window with no problem. But I'm having trouble removing the adhesive. I don't want to use a razor blade on the rear window, since it will probably break the defroster lines. I tried soaking the adhesive in an ammonia based window cleaner, but it didn't seem to have any effect on dissolving the adhesive. Even tried using a heat gun, but it didn't help. Plus, I was afraid of cracking the glass with thermal expansion of a section.

Any ideas or tips for removing the adhesive without damaging the defroster lines?