Two wires go down to the trans switch. In and out. That is it. Power goes to the switch- when put in reverse power goes thru the switch and back to the harness and sent out to the rear lights.
Not much more than that. Why 4 wires?
Wrong harness? Sounds like a console harness for a automatic you have.
This was purchased thru megaparts and the listing said it was for a 71 dart 4 speed. I did call and ask if it would work for a 72 but they couldn't give a definite answer.