Get a spray bottle and mix warm water with amonnia. Spray on and let it soak for 10 minutes or so, then wipe with a cotton fiber cloth. It will come off much easier. Cover the rear deck with a garbage bag, so you dont get it all wet with the spray. Its not an easy job, or fun, but when its done, your like wow, I have a clean clear window.
I did it on my wrecked 96 when I got it, cause the tint was so bad you could barely see on a rainy foggy night.
Dan