Diamond Shield is not a 3M product. Mine failed too. Its a DOG to remove. I got my lower off with scrapers I got from a window tint shop.They also have a product called TINT_OFF that cleans up the glue. I did it in summer in Florida. I have read a wallpaper steamer works better than a heat gun. I could get 10" square chunks to come off by getting a corner and slowly peeling out. The area above the windshield must have been a different batch or material as it turned almost black and is/was a nightmare to remove. It is coming off in dime sized pieces. I am still removing it. I need a scaffold. The tint shop quoted $600 to remove. It may be worth it. You are lucky if paint doesn't come off and your decals if any will probably need replaced.
Very unhappy customer here.