What vehicle? As mentioned, if good reproduction ones are available, cut the old one for ease of glass removal. Back glasses are tempered and pretty tough though. If your seal is still pliable and reusable, working it and the glass out of the frame is not all that hard on most models. Just don't pry on the glass with hard tools. I like to work the rubber gasket over the frame lip with a putty knife while giving the glass a slight steady push with my shoulder.