Instead of solvents, there is a tool for that. It is a rubber wheel that goes on a grinder or power drill. You get it spinning at high speed then lightly touch it to the decal and it peels off clean with little or no residue. We used this in prep work at the body shop where I used to work and I took a lot of decals, old dealer stickers and aftermarket graphics off of vehicles without a single scratch. A light buffing with a polish (not rubbing compound) will even out the luster and get rid of the shadow effect left behind by the decal. If I remember right, the wheel is a 3M product available anywhere you can purchase automotive finishing products.