73 Duster electric (element) rear defogger
The newest manual I have is 72. There was no heater in the defroster that year, only a 2 speed blower, fed by no18 wire to the rear. There is a resistor back there, for the low speed blower
The 72 service manual:
http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download
I could not find a blower diagram for a Valiant series, I believe the blower is out of the Satellite section. The wire colors are different, but the hookup should be the same.