Wiper speed?

Not that rock auto is the be all end all of parts but they only show a 2 speed motor for 71 duster.

The 71 dart wiring diag only shows one configuration for the wiper. Same for the valiant. But the switch configuration is different between the diagrams????

Perhaps the switch is not the correct one for the wiring harness, or perhaps the wiring harness has been swapped?

You could apply 12v to the correct pin of the connector and see if the issue is the motor or the switch /wiring.