White spirit can be effective.
If you lift up part of the film and brush some white spirit in, it will soften most adhesives but isn't too powerful on paint etc.
Another effective method is to use a hot air gun to soften it up.
I worked for a company that liked to change it's looks every couple of years, so all the liveried vehicles had to be re-done.
The people doing the large fleet used heat guns to remove the old film