Heater Fan not working

72 and back fan switch moved left to right and did have an off position.
I'm going to guess your 73 has a fan switch the moves vertically. There is no off postion in that switch. It does select which resistor(s) the current passes through to create the various speeds.
To get the fan to go off is done with a disconnect in the switch which is operated by the upper sliding lever. That switch is both electrical and vacuum controlling. Most liely culprit too. There is quite a bit of current passing through those contatcts. You may find melted harness connectors there.
I have a good used switch if yours is factory a/c equiped. There is a bit of labor involved to R&R that switch. A new one will likely be a chinese repop but still, Who knows how long a used one will last.