Tinting a curved surface is difficult, but not impossible. If you're only doing the rear, then you should have a couple shots out of a roll. There are some good video's on youtube showing how to go about laying out, shrinking, and applying film to curved surfaces like that, I would take a look. You can shrink with just a blow-dryer, you don't need a heat gun (though it does speed the process up).
Just ensure that you don't knick the defroster lines with anything sharp and you should be good to go