1970 Duster heater fan only one speed works
Most likely right in the control switch. The "starting point" is to wander over to MyMopar and download a service manual. The most of the info you need is in the AC / heaters section and not the electrical section
http://www.mymopar.com/index.php?pid=31
Also wander around in the "Master tech service conference" bulletins there might be some good info there
http://www.mymopar.com/index.php?pid=446