Window tint has evolved, and replacing old tint adhesive is a common automotive repair. Old-style tint adhesive that used to turn purple and bubble off the glass has been replaced with longer-lasting films. Removing the old adhesive is actually a simple process; it just takes time and patience. The backyard repairperson can remove tinting film from several windows in about two hours.
Spray the inside of the window with the ammonia-based cleaner until it is saturated. Normally, it is advised that ammonia cleaners not be used on the inside of tinted windows, because it will detach the film from the glass. The higher the ammonia content of the cleaner, the more it dissolves the tint glue.2
Separate the film from the glass around the edges. Find the edge of the glass and run the razor between it and the film, spraying the area to induce separation. Ideally, about two inches of separation around all edges of the glass is enough to get a grip on. Realistically, the film will come off in small slivers that will require diligence to peel. What is needed is a "tab", or section of film that can be pulled and sprayed to remove the entire primary film layer. What is left after pulling the film free is a layer of glue that has bonded to the glass over years.3
Spray a liberal dose of glue cleaner onto the layer of glue left on the glass. Using the towel, rub the glue into "streaks", and then dose the area with more glue cleaner. Eventually, there will be a small glob of glue in one patch, and it can be scraped off with the razor blade. Several globs may occur, but using the towel to apply friction saves time versus trying to scrape the window with the razor alone.4
Clean with the window cleaner once more. With the glue mostly gone, the ammonia based cleaner should restore the window to a pre-tint state.