I would try a plastic scraper. If that does not work then maybe move to a spackle or putty knife. Nothing too sharp but nothing too dull. I had the same issue with the '58 Fuelie I worked on months back. Half the under-mat came up and the other half was still glued to the floor. Also try a little paint thinner. Paint thinner will not damage the glass in any way. Scotch brite works too. Try the red or abrasive pad. I think that should do it... Oh, when you're all done, Wax and Grease mover (Prep's-all) will take care of the rest.
-Dan
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