If you purchase a used vehicle that has had improperly applied tint or if the aftermarket tint on your car is beginning to fail and separate then you will need to remove the old tint so you can apply some new.
This process can be very messy and because you will be using ammonia can cause you discomfort if you have breathing problems. I suggest that you contact a local shop that specializes in installing tint and ask them the price to replace your tint. Since they perform the work every day they are likely to do it fast and correctly.
If you want to complete the work yourself then you will need to take your time and work with good ventilation but the tools needed to perform the work are pretty basic and you may have everything you need in your house.
First you must understand that window tint once applied has a plastic layer that faces in to your passenger compartment and a inside gel that is in contact with the window surface.
As window tint begins to fail the metals in the tint may begin to turn purple or the tint may begin to lift in areas. Unlike your initial application once the tint begins to lift later in its life you can not just pop a hole and squeegee it back onto the window. Even if you could re-adhear it the amount of area would be too much to fix without destroying the outer protective plastic layer.
Once your tint has failed pulling the plastic inside layer off will cause the gel coat to remain on the window. You will need to heat the tint and apply ammonia to help release the tint and plastic layer in one piece. If you can not work with ammonia then see the steam method below.
It is suggested for best results to use both the ammonia and steam method combined.
First you will need to cut a black plastic bag to place on the inside of the window to keep the ammonia moist while the window heats in the sun. Start by placing it over the outside of the window and cut it to size. Spray the inside of the window with ammonia and then place the plastic you just cut over the wet window. Let the car sit in the sun for at least an hour to warm the tint.
Place another plastic bag or plastic tarp on the inside of your vehicle to protect your interior. Ammonia is very strong and could bleach out or spot your interior if it comes in contact with fabric or other materials.
Spray the tint with ammonia and then to reduce evaporation cover it with a plastic bag. If you do not cover it you will need to respray the surface to keep it wet.
Steam Method
A second method of removing the film is to heat it with a personal size steam cleaner. This should heat the material and loosen it at the same time.
Work very slowly by testing one corner. If the gel and plastic backing separate then you need to apply more steam for a longer period of time.
The good thing about this method is that it does not require ammonia. The bad thing is your results will vary and it may take a little longer.
If you are removing tint on a rear window with defroster lines you really must use the steam method.
Never try scraping the rear window or you will damage the defroster lines.
Side windows can be scraped but you risk causing damage to your window.
To remove gel coat that remains behind soak the area with soapy water until it loosens.
The use of caustic liquids like carb/injector cleaner or break cleaner should not be tried especially on rear windows with defrosters.
Final Note
A good steam cleaner and a pre soak with ammonia in the sun is probably your best bet.
This job is neither fun or easy and will take you a number of hours to complete.
For this reason it is important to keep the area wet then work slow as you remove the tint.