From http://doityourself.com/auto/automotivetintremoval.htm

"You are more than likely to damage one or more of your defroster lines, when this happens, the cut line will not work anymore. Some cars have radio antennas mounted alongside the defrosters made of the same material. If you damage these, your radio antenna will not work! Be careful! Taking it to a tint shop to be done greatly increases the odds of saving the lines, but even a tint shop can not promise to save your lines, sometimes the film is just to far gone."