The rear window is held in by a rubber flap over a short piece of body metal surrounding the entire rear window opening. A small amount of careful prying at the lower corner of the drivers side resulted in the whole window assembly moving slightly. I have my mostly-useless rear quarter windows removed, so it was simple to reach in and using the palm of my hand, to push & work the assembly into the cockpit. Be prepared: the assembly weighs 10 lbs and is 5 ft long. Its not easy for a single person to manipulate the thing without breaking it. Wilkinson has the gasket you need.
Josh