Hello: I believe that the back window in the 68 Crown Convt was a type of plexiglass. I had one new as a Company Car. It was Turquoise with a white top and white leather interior. It was a looooong car. I believe that there are folks who can fit a new top and save the original back window. I have no idea where the car is located but the guys who do the tops for the show cars can do almost anything. There is a guy in Minnesota that used to go under the name Ed's Upholstery that did most of the show GM cars where you really needed a first class job. The other point that is important is that the original window is date coded and when it was destroyed you would have difficulty finding another. Darrell Davis