I have mine stuck to the front of the glass. Not had any problems. You will really need to go some, to damage it (how often do you touch the glass anyway) and it will stand up to any cleaning, rubbing, general knocks etc.
I got all my art done at mamemarquees and have been very impressed with it. Its very durable, and hasn't faded at all (one side of the cab is next to a back window in direct sunlight for about 5 hours a day).