I replaced both of my rear quarter panel window seals. Taking them out is easy Putting them in requires a little bit of string and some help from a friend.

Taking them out...just work a portion of the lower inner seal up over the window frame. Its not that hard to do and doesn't damage the seal...the window works out pretty freely after you get a good portion of the lower seal over the inner window lip. I recommend using a paint scraper with a bit of duct tape over the end so as not to damage the seal. Have a friend pull out and down on the lower part of the window as you work the seal off from the inside. The window will come off.

On putting the window in, put the seal on the new window and the run a piece of string around the window, in the inner track of the window seal. Then place the window in the frame with the string ends on the inside. Work one edge of the seal over the frame lip where one end of the string starts.
Slowly pull the string along the window frame with your friend applying pressure with you as you go. The string will pull the seal in over the window frame lip as you work around. You can see it "suck" in from the outside. Sometimes it takes a couple tries to get the seal to sit right but the string trick works beautifully.....

but I cant take credit for it...someone else on this list taught me...

good luck
'89 928S4, 86.5K miles in NH