Use a razor blade in an inconspicuous spot(like top left or right corner). If it flakes, it’s more than likely a laminate layer(film) and not glue. Don’t use the blade over the rear defroster lines*. If it’s determined that it is a layer of film, take a piece of scrap film or the cheapest line you keep in stock. Cut and heat mold as you normally would. Install it as you normally would. Squeegee out as you normally would. Now take a black trash bag and cut it to fit on the outside of the glass. Take trash bag from the outside of the glass, and using slip solution(soapy water), install that black bag over the newly tinted window. Back the vehicle out of your shop and park it in the sun. Make sure the back window is getting full sun. Wait about 30-60 minutes, until the glass is so hot you can only leave a finger on it for less than a second. What you’re doing is using the trash bag as a steamer, steaming the entire piece of glass at one time. The newly installed piece of film *should* bond to the old layer of laminate, and it will all come off in one shot.
if that doesn’t work, get the ole scraper out and begin to go to town. Let customer know he/she will no longer have a functioning defroster. Use soapy water so the adhesive doesn’t get soupy and stuck in the rear deck or c pillar cloth.