?? On neutral safety switch wires

I ran into this recently when I added back-up lights to my 69 , in the end I found an odd shaped three prong connector on the harness that crosses the firewall that the sub harness { the three wires your speaking of } plugs into , my confusion came from having a 68 diagram which is completely different for this circuit , I too couldn't figure out where the 2 outer wires where supposed to run as my mechanic direct wired the center { neutral safety switch } wire as a temp thing until I found some back-up lights , the connector was about mid-way between the distributor and the terminal block on my car perhaps it is buried/taped up somewhere along your firewall