Rich,  The switch on the console feeds a heater fan relay  in the engine compartment, thats what you hear clicking.  Not sure where it is on a 70 but on my 82 it is mounted on the firewall between the distributor and the passengers side fender. In additon, there is a heater fan resistor which is mounted under the hood.  The relay on mine is mounted on the passengers side actually inside of the fiberglass air box.  The base of the resistor is a kind of diamond shape and is secured with two screws.  There will be two wires going to it.  It is quite large when you pull it out of hits hiding place.  Your trouble could be in the resistor.  I think the resistor handles speeds 1, 2, and 3 and the relay handles the high speed.  If you will trace the the wires from the fan motor you should find the resistor and further tacing will lead to the relay.  If you find burned contacts on the relay, they can be carefully cleaned with fine abrasive paper slid betwen the contacts  to smooth the burned spots.  If its the resistor, I dont know if there is any way to repair the old one but new ones arent too expensive, in the $20 - $30 range I think.

The blower not working on Low speed is usually the resistor, located on/in the A/C plenum box, under the hood. The "clicking" you hear when you try to use high speed is probably the high speed relay, also located under the hood, by the A/C box.
I see you have two cars...which one is the "problem child"?

oops, my fingers got ahead of me.. the RESISTOR is in the air box, the relay is on the firewall...

Thanks guys for the quick response.  The problem child is the 75.  Of course I saved the electrical issues for last. :)