Goo Gone, WD-40, naptha/lighter fluid. They all work on old tape residue. Just give it plenty of time to soak and loosen up the crud, so you won't need to scrape with anything harder or more abrasive than a plastic tool -- like a plastic picnic knife or popsicle stick. Might take a few days. Then isopropyl alcohol to clean off any residue from the solvents.