Heat does work sometimes but usually involves a little scraping. For scratch free pinstripe removal you need a large decal eraser. You can pick up something like this at any automotive paint supply store and some NAPA's carry them. Wash area before using this.
Attach to a drill and go to town. Works everytime.
You will also be able to see where the pinstripe was due to paint fade, etc. Some fine compound and some elbox grease help out. Don't forget to wax then.