Instrument gremlins.
Look out for corroded contacts.
And as for the heater question, there are variables. The heater may stop working due to these possible reasons:
Bad fan motor
Bad fan speed switch
Bad resistors
You can check the blower motor by momentarily linking a hot 12 volt line to see if the motor kicks on. (again check the wiring diagram to make sure you don't fry stuff).
Here is a Wiring diagram for 75 Valiant And a 73 Valiant in color: (The wiring color code should be similar)
This is the color coded one: (Large size picture) http://board.moparts.org/ubbthreads/upload2/6493035-WiringDiagramFullValiantDuster.jpg
Look out for corroded contacts.
And as for the heater question, there are variables. The heater may stop working due to these possible reasons:
Bad fan motor
Bad fan speed switch
Bad resistors
You can check the blower motor by momentarily linking a hot 12 volt line to see if the motor kicks on. (again check the wiring diagram to make sure you don't fry stuff).
Here is a Wiring diagram for 75 Valiant And a 73 Valiant in color: (The wiring color code should be similar)
This is the color coded one: (Large size picture) http://board.moparts.org/ubbthreads/upload2/6493035-WiringDiagramFullValiantDuster.jpg