With the engine cooled off, hook up a cooling system pressure tester. Watch for leaks. If it's leaking at a hose-to-radiator-pipe-connection, gradually tighten clamp until it stops. You can rent these at the mass marketer parts shops like Auto Zone. The one they rent fits perfectly to my MBs pressurized coolant tank without any adapters. I've found these to be invaluable for locating leaks. Much better than trying to eye ball a warm/running motor. That approach doesn't always work.
Another possibility is a small plastic elbow piece on top of radiator(more to drivers side) that has a long/skinny hose connected to it. This hose goes back to the overflow tank. This elbow has been known to leak and it's been mentioned at this forum many times that it's replaceable and available at your local dealer. It's also available here. Photo below.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1KU172UOY1WE0NEJEJ&year=1989&make=MB&model=300-CE-001&category=G&part=Radiator+Fitting