The problem with removing perished window tinting film is that the tinting film actually consists of two layers of tinted polyester. What happen when you try to peel the window tinting film off the glass is the two layers will tend separate in places, with the top layer coming off while the second layer may remain behind. The second layer will not peel off on its own but will flake instead, and will only come off if scraped with a razor blade. When using a razor blade to scrape off the polymer film you run the risk of cutting one or more of your defroster lines and a cut defroster line will not work! The trick to successfully removing the window tinting film is to remove both layers at the same time. This is how to remove window tinting film with ammonia based cleaner.
First, cut a plastic garbage bag open so that it will cover the car window. Then spray some water on the outside of the window and place the plastic garbage bag over the window. Now cut the plastic bag so that it fits the window.
Next, remove obstacles that may get in the way or that is against the glass, including the third brake light on the rear screen and the rear deck. Then cover the car seats with a tarp or some clean plastic garbage bags.
Next, liberally spray undiluted ammonia based cleaner directly onto window tinting film and cover the tinting film from the inside with the plastic garbage bag you cut to size earlier. This will prevent the ammonia based cleaner from evaporating. Now park your car with the window in direct sun and leave it for an hour.
After an hour the window tinting film should be ready to peel off. User a razor blade to cut a starting point in a corner where there's no defroster lines and try to peel the tinting film off in one go. You could leave the plastic garbage bag in place so that the ammonia based cleaner doesn't dry as you peel the tinting film off. If the ammonia based cleaner does dry, the adhesive will harden again.
When you've got the window tinting film peeled off, you will need to scrub the remaining adhesive residue off using some ammonia based cleaner before cleaning the window with window cleaner and some paper towels.