Help, 72 duster NSS/Back up light harness hook up.

Mine is the original 904 transmission. It does not have a seat belt warning light. It has the wire to the starter solenoid, already hooked up. Then a 3 prong plug in and everything ive seen says it plugs into the bulk head, But i just can not find any 3 prong plugs on the bulk head. I will look again tonight and ill look at my head light harness and see if there are open spots to hook up like 6pk2goDemon.