Aux fan switch is on the reciever/drier of a/c system.. the one with the pigtail leads ..

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1D00YU6RY1JK12H7B3&year=1977&make=MB&model=450-SL-002&category=R&part=A%2FC+Temp.+Switch&appChassis=_any

Just run a couple of wires from these to a cabin toggle sw..

Jumpering these leads will also verify the fan circuit.