I've come across some fairly detailed explanations of how to do this on posts here in the forum. Something about using a string/lanyard inside the gasket to hold it open in place, setting in the glass, and then pulling out the string. Hopefully a search here can pull up some of this info for you.

My brother has a client that is a glass company, including lots of automotive glass. They told him the Mercedes W123 is particularly difficult to do, and that the rear glass is subject to breakage during the seal replacement.

This summer I'm having both the windshield and back glass seals replaced. They both leak a small amount. I think this is a good investment in the life of the car (rust prevention) and new seals should last 15+ years.

Are you doing this before or after the paint job? I'm thinking you might want to have the glass out for new paint so it covers the metal under the seals.

Ken300D