How do I remove rippley purple window tint from the back window of my 1991 318i? Apparently it has been on for at least a decade and maybe two. I try peeling it and it comes off as two layers, both of which have the purple tint. Neither layer is soluble in water, mineral spirits or acetone. The tinting material has been applied as three 8" wide bands horizontally across the defroster wires.