Try this?
I've used 3M's decal and adhesive remover with great success in my shop. There are a host of other chemicals or products that can be used with proper techniques, however they can damage the paint, clearcoat or finish if not done properly. Things like lacquer thinner, Ronsonol lighter fluid, DuPont surface prep or other solvents require varying (but high) degrees of care and also typically require some restorative processes (buffing, polishing, etc.) afterward. Before you even think about trying these, you need to discuss the matter with your local automotive paint jobber. They are typically reserved for more of the "oh S@#$" cases where the only other viable step is reshooting the panel.