If applying heat doesn't work (it may not because glue gets really hard after a few heat cycles from the motor), I have had good results by:
1- Lightly sand the paper coating with 220 grit or finer paper just enough to scuff only the decal surface. This will make the surface porous.
2- Spray or using a cotton ball, dab tar remover onto the paper surface. You can get tar remover in any auto store.
3- After it soaks in for a few minutes try to lift the edges and work off.
4- Finish off with soap and water.