From what I have been told from a tinter is take the window off the car and use a razor blade and what not to peel the tint off. Once that is done you have to soak the window in some sort of solution over a period of a few hours to get the gunk off the window that was left behind from the tint. It's a PITA to do, if you can I would wait till spring to do that, or bring it to a tinting place and have them do it. If it's tint all around, you want to remove, be very careful when taking the tint off of the rear window, you don't want to damage the defroster or radio antenna in the glass.
1994 318is-Sold July 13th 2010.. Sad Sad Day