My vinyl guy had to resticker one of the race cars. Had a normal, Wal-Mart type hair dryer in his tool box and used that. Told me that a heat gun can get
too hot which creates all kinds of problems.
He also turned me onto 3M adhesive remover. It isn't cheap, about $19 for a large spray can but it
really works great. A small amount goes a very long way to get all the adhesive that is left over. I would think that any vinyl cutting shop, decal shop, auto body supply shop, and maybe even an auto parts store would have it. It has a citrus smell to it. I've used it many times and it works. You just have to remember that some of the adhesive, stickers, and decals come off harder than others.
Then he clued me on the final step ... isopropyl alcohol. Buys it at Walgreens in the big jug and puts it in a medium sized spray bottle. He cleans the entire surface where the new vinyl is going to go down. He says it takes off any residual oils (like from your fingers), things from the air, 3M adhesive remover, etc. and then any of the alcohol left quickly evaporates leaving the surface ready for the new vinyl.
i have had some success using goof off for that purpose.
2x Goof-Off for the hard to get stuff. I think its expensive when you have lots to do.