All window film deteriorates with age and eventually has to be removed. Two of the most common symptoms of dying film are purple film and bubbling film.
Purple film is caused by non-metallic dyes in the film breaking down and changing color. Bubbling film is a sing that the adhesive used to apply the tint to the window is drying out. Once a single bubble appears, many more will follow.
It is recommended that you have the window tint removal performed by a trained professional. The process can take up to 3 hours and is not easy or pleasant. Improperly removing window tint can damage defroster lines and radio reception. Many tint shops will include the removal for free with a new tint job.
If you decide to remove the window tint from your car you will need black trash bags, undiluted ammonia, super-fine steel wool, razor blade, glass cleaner, at least 3 hours of your time and a nice hot sunny day.
First, cut the garbage bag in roughly the shape of the window, spray soapy water on the outside, cover it with the black trash bag and smooth the plastic flat. Most window tint is constructed of multiple layers of film. When left under the hot sun, the garbage bag will absorb heat, helping the film peel off in one piece.
Protect all inside surfaces, such as speakers, rear light, and upholstered surfaces, near the rear window with a tarp, then spray the entire window film with undiluted ammonia. Undiluted ammonia has a strong smell. It is recommended that you hold your breath or wear a facemask.
While the ammonia is still wet, trap the ammonia against the window film with another trash bag or plastic wrap. Park the car with the rear window facing the sun for an hour to soften the tint's adhesive.
You can now start peeling the window film off. Use your fingernail or razor blade lift the window film in a corner of the window, and try to peel the film off in one entire piece. Keep the window tint moist with ammonia as you strip. Use a razor blade to scrape off any tint that did not peel. Be careful not to cut the defroster lines.
Remove any residual adhesive with ammonia and Super-Fine steel wool, wiping with paper towels before it dries. Remove the exterior trash bag and clean the window thoroughly with glass cleaner.
Resources:
WWW.HHP.UFL.EDU
WWW.TINTDUDE.COM
WWW.EXTREMEAUTOACCESSORIES.COM