Are these the original stripes on the car, or have they been replaced at one time?
Old vinyl, and the adhesive, turns very brittle and dry with age and is notoriously hard to remove.
I hardly ever use a "heat gun" for removal. I have a 1200W compact hair dryer that works fine without burning or stripping the paint or bubbling filler, etc. With the dryer, the air is not hot enough to burn your hands, either. You can work in the airstream easier while keeping an even heat on the decaled area. Definitely do the work in a heated garage (this time of year).
Also refrain from using a scraper of any kind if possible. It WILL cause more damage than you bargained for. I will use a DULL, UNDAMAGED single sided razor scraper JUST to get the lift started, then keep the heat source just in front of the stripe as not to "melt" the "string" of vinyl and break it. Pulling the vinyl this way will remove the adhesive on the vinyl in most cases. If you scrape, the adhesive will tend to stay on the substrate (paint), causing more work.
It takes practice and a lot of patience.
Follow up the adhesive removal with a WET soaked rag in mineral spirits. Work small areas at a time. A damp rag won't do. I will then use the same dull razor or plastic scraper on the swelled up "jelly like" adhesive. Yes, it will be messy, and usually require removal in layers.
Good luck, and work SLOWLY.
