Mercedes: 1982 300D rear window replacement, paint paddle, rust repair

Question
How do I install a rear window on my 1982 300D?

Answer
If am assuming that you already have a replacement window. I believe that the gasket has a rubber key in the center. If so, find where the ends meet and using a narrow object, pry up the key. Once you have the end out work it all out around the complete gasket. Once the key is removed you should be able to push out the rear window from inside the back seat.
Clean the gasket thoroughly. line up the center of the top and bottom make sure you install it just as it was on the old glass. throughtly clean the window frame. If you have rust repair, prime and paint and spots. Once you have the opening repaired, insert a small rope in the gasket grove.
With an assistant set the glass with gasket into the opening on the outside. Have the assistant hold the glass and using a paint paddle or similar object get the gasket over the car frame edge. As you work the rubber into the frame peel the rope from the gasket that will help roll the rubber into the inside of the car frame.
Once you have gasket completely installed you should probably seal the gasket and then reinsert the key.
Using a water hose, check for any leak and caulk as necessary.
Good Luck.
Now if this window is glued to the body opening with a cover molding,the it requires a piano wire drawing along between the body and glass to cut the seal. Once you have the old glass cut out, you will need to thoroughly clean the body opening and remove all traces of the old sealant.
The new glass will need to be leaned around the edge with alcohol, just before install to remove any trace of oil, grease of contamination. You will need a special tube of windshield urethane sealant. It often require a special gun with a mechanical advantage.
Follow the mastic direction for installing a glass.
You will lay down an adequate amount of sealer. With the help of an assistant you will set the glass on the opening  using positioning block to keep the glass from sliding down.
Once installed you will need to press frimly to make sure that the sealant has adhered to the glass and body. Be sure to not allow the glass to set so deep that is you squeeze out the sealant and the glass is touching metal. Clean any gasket with the proper cleaner. Be careful not to allow it on the paint as streaking ma occur.
Good Luck
Rob