i done this a couple of weeks ago and tried a few things,
i just peeled off the tint film quite easily using a blade to get it started at the top of the window. then i think it was a water based glue underneath which was quite difficult to remove when i tried brake cleaner and ag intensive tar remover. ended up using white spitit on a cloth to soften up the glue then use a blade to scrape it off, it took me about an hour to do the two front windows of a transit. a tip is to keep using new blades after a while because when they get blunt they miss bits when you are trying to scrape