Wiper speed?

I don't think that connector will go on but one way. They didn't key their harness connectors often. The round one at inst' panels is keyed. Most are just shaped or arranged to go on only one way.
That terminal could be for additional single wire connector. Washer pump? CB radio or other option? They picked 12 volts off the wiper switch for reverse lamps in some models. A manual might reveal that every car like yours has one unused terminal on the wiper switch.