Need help with 73 duster heater fan problems

You should be able to jumper across the resistor terminals at the harness connector, and if the switch / harness is good, it should run "on high" at all settings of the switch. If it does, it's a resistor issue. If not, a harness / switch issue.

Are you sure it's not a MOTOR problem? If the motor bearings or blower wheel is dragging, it won't (might not) run on the lower speeds.