Rear Window
You are talking about the fastback glass? ...not the back seat window.... right?
If so, I talked to a guy in southern Indiana about a year ago, and he had a rear '65 Cuda glass........ he asked me if I (or anyone that I knew of ) needed it. If you can't find one, let me know.
I'll have to scrounge around to find a way to contact the guy, but I didn't get the feeling that he wanted an arm and a leg for it.