Heat gun or not, really doesn't matter. If you get some amonia, spray it on, then put a trash bag against it to keep the amonia from evaporating so quick. Let it sit in the sun for a while. Then use razor blades. Once you peel film off, it's easier to remove the glue that's left with soapy water and another razor blade. Be real careful with the back window if you have a defroster. I'd let the amonia/trash bag sit for a couple hours on the back window......but as careful as you are it's very likely that it'll never work again.
