Have never had to do it myself, but if you start with a knife blade, cut and peel back a corner until you have enough to hang on to and then gently peel the tint off. Ant residue should be able to be cleaned off with Metho.
As it is your rear window, be VERY careful around the demister lines that are stuck to the glass. If you damage these, your rear demister may not work properly (or at all, depending on which line you break) any more.
Dan.