when i did my tint I removed the windows from the car. It is much easire to do the tint that was and that is the was that most shops do it,
It really depends on how far down the orignal tint goes on the window. If it goes below where it is visible I would say to remove the window from the car. You cna retint at this step too.
It sounds like your back window was doen in strips, a common way to do a large back window. You can remove the tint by peeling the strips off, it will net hurt the defroster lines. DO NOT use a razon blade on the back window to remove the tint. that will scratch the deforster lines and cause them not to work