Unfortunately, right at this moment I can't come up with a satisfactory "solution". A few years ago, I became involved with the removal of tints from a Nissan SUV, and it took a long, and frustrating time. I obviously used the wrong material - whatever it was. I used a heat gun, my finger nails , God knows what have you... but it was not a nice job. It is only last week that I managed to find an experienced Windscreen Remover/Installer, and I'll contact him for any procedures/materials he can recommend. Watch this space. Regards Styria
N.B. Hope I won't be too late - BTW, I did witness old film removal a couple of years ago by another Installer. Whatever he used, it all came off easily.