Disconnect the two wires at the temp sensor on the receiver/drier. Then connect the two wires (the ones not attached to the switch) together. The fan should run when the key is placed in the "run" position.

The temp switch looks like this:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2650Q36L326X0CCT0A&year=1983&make=MB&model=300-DT-001&category=R&part=A%2FC+Temp.+Switch