I bought some of those trailer wiring harnesses from the local Advance auto parts. They sell 2, 3, and 4 pin extension cables for trailer wiring. Cut it in half and you get the male and female for $10. They plug in fine with the connectors on the 73 mustang.
Even the square 4 pin connectors under the hood at our fire wall. I used the 2-pin trailer wiring harness to plug into 2 of the pins in that square to create the jumper for the neutral safety when I converted from automatic to manual transmission.