I have a 1981 240D (not a 1980). I have a strange problem with the fan blower. The fan only works when the key is turned to were you can listen to the radio but the engine isn't runing. But when you start the car the fan blower won't work. What could this be?

It's possible the electrical portion of your ignition switch is bad. Can you get the fan to work by wiggling the switch back and forth a little when it's running? I had a similar issue on my 300D, the fan would only work if I turned the key slightly back after the car was running. To fix it, I replaced this:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41VC18CFB0&year=1981&make=MB&model=240-D-002&category=All&part=Ignition+Switch&appChassis=_any