Well, I would try all sorts of stuff. I would try a heat gun, various rotary devices guys have mentioned, a sharp putty knife, etc. etc. Also, if the bond between the old liner and the bed is weak you will be surprised what a car-wash high pressure hose might do--(took the paint off the top of my cab with one on time!) But I would try them all, and then just buckle down and go with the one which works the best.