Like John Rist said, the trim solvent works to clean it up.... But also, if you "flood" the edges a little with trim solvent it will wash away the adhesive on the ends and get at least a corner to raise. Then keep wiping at it, under the lifted corner with a paper towel wet with trim solvent. You'll slowly wash away enough adhesive to lift it away. I've found the trim solvent works best for removing really stuck on layers. You might even have to pull up the raised corner with tweezers to keep wiping. It certainly can be tedious. Especially if the last guy did a great job... Lol