Don’t use heat at this point, heat is to removed the decal, at this point all you have is the adhesive. I use a product called Rapid Remover, it’s geared for sign/wrap industry not sure if you can find it outside of suppliers.
Either way. The key is to really soak it. One trick is to place a paper towel flat over the adhesive and spray you choice of remover on it. This will cause the paper towel to cling to that area. Keep soaking the paper towel, the trick is to lock in the solvent and let it really do it’s job. Once it’s been good and soaked remove the paper towel and take a clean rag and rub. It will ball up and will be able to get it off. Repeat process again if it’s being stubborn.
This works a lot better than just spraying it on, as most will just run off the adhesive. A lot of removers will also evaporate quick…