Sounds like a bad blower regulator if it only works in defrost. Do you get air in other settings if you set the fan switch to max?

The blower regulator is beside the blower itself, under the hood -

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=28G13A2TU28G13AB1Z&year=1998&make=MB&model=S-320-001&category=R&part=Blower+Regulator

Sixto
87 300D