Not true. Defroster grids barely get hot, not near as hot as a window gets setting in the sun during the summer. Bubbling will not happen if the tint is installed correctly and the surface is properly prepped. A good tint shop will guarantee against bubbling. Cheap tint does turn purple and the more expensive brands like 3M are guaranteed not to turn purple. However they all fade with time. I had 3M on my truck and when I took my Oakley sticker off the back window you could see a darker area where the sticker used to be.the reason it bubbles up in the back window first is because of your defroster. If you turn it on and leave it on it will cause the tint to start to come off.