I just took the rear glass out of my MK2 golf, I probably got lucky too. My method was: open hatch, find 2x4 that touched the inside of the glass and the ground with the hatch horizontal, put almost full body weight on hatch near where the 2x4 touched the glass. It worked surprizingly well and came out unbroken and I'm pretty sure the seal is fine too.
You might want to go around the seal with a plastic scraper and loosen it from the body, it's no doubt stuck with age.
For installation: put the glass in the seal, wrap a clothes line a few times inside the seals groove and with a helper outside holding the glass against the opening and you inside, pull the clothes line toward you. This should flip the seal over the pinch weld.
It'll take a few tries but it works, I did it on a vanagons side glass, go easy on the seal it'll try to tear if you're too hard on it.