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

That one is there and plugged in. There is also a 3 prong plug that comes from the transmission along with the wire to the starter relay, that is plugged into the NSS. I can't find where to plug that 3 prong plug into. Im assuming its mainly for my back up lights because they are not working. But the NSS is working because it will only start in park or neutral and i just replaced the NSS. Im trying to get my lights working. Any ideas on the plug?
Thank you for the quick reply though!!!