What year Mustang, someone wanted me to do their 95-98 ish Mustang and I couldn't because most of the rear fender was plastic...and the part that wasn't would have been worthless to roll. I was told by a shop they usually have to cut those fenders for it to work.

Also, it is hard to screw it up w/ a heat gun and some patience. Figure out how the thing works before you start doing anything and go slow. Cars w/ flairs already...IE your GTi are a little trickier then, say, a 240, so make sure it is at the right angle and no part of it is hitting anything other then what you want moved. I practiced on an E30 and messed up the first one a little bit, luckily it didn't matter, but it is definitely different then a flat fendered car.