Lighter fluid works well for dissolving the gummy adhesive left behind. That's what we used in the sign shop I used to work in to get pin stripe adhesive off before applying lettering to the Police cars. The hair dryer is the best place to start. You should be able to work a corner free and pull it off by hand slowly as you heat it with the hair dryer, then put lighter fluid on a terry cloth rag to remove the adhesive... You will have to be generous with the lighter fluid, and turn the rag frequently since the adhesive goos up the rag pretty quickly.