If you are getting water on the drivers side replace this seal:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1MB12FET61NN1CBMXU&year=1987&make=MB&model=300-SDL-001&category=Z&part=Antenna+Seal
It costs nothing takes a few minutes and is usually the suspect.
Water can also come in from behind your tail lights.
Now water should not come in from the rear glass regardless of the glass or its seals condition. Their is a metal lip under the glass that will not let water into the trunk, unless it is rusted through. So if you are getting water in your trunk from the rear glass you have a hidden rust problem. I sudjest pulling the glass and dealing with it if you plan on keeping the car.