How to remove Mustang decals from side of car?

A good hair dryer will work, a heat gun is better but the chances of blistering the paint is also much higher. 3M used to make a release agent. I don't know if they still do or not. Though it worked, I never thought it worked all that well. A supply of new, good quality, sharp, single edge razor blades and some soapy water (50/50 mix) in a spray bottle, and tons of patience works the best. I used this to remove the bird decals from Trans Am's. Also It will help a lot if you can let the car set out in full hot sunlight for a few days when temps are in the high 80's or hotter - the hotter the better. This tends to soften the decal some what. Start on a corner, spray the area with the soapy water, and using the razorblade, very gently begin working your way downward. When you had detached enough of the decal to hold between your thumb and forefinger, pull very gently with one hand and use the razor blade with the other. If you are a hunter it is much like skinning a deer. Don't try to do too much at a time. and always keep the top of the razor blade as low (next to the finish of the car) as you can. Use plenty of soapy water. This keeps things slick and you are less likely to gouge the paint of your car. But patience is the key. Good luck.