neutral safety switch - 1968 383 4spd Barracuda

You talking about stick or automatic? Stick shift cars did not have a clutch switch until 70

Go to MyMopar and download a factory service manual free, and also the aftermarket wiring diagrams there, which are sometimes easier to follow but not as detailed. I believe the backup lamp power originates from a half moon shaped connector in the instrument panel harness. This goes to a column switch or the console for automatics, and somehow feeds through the wiper section of the bulkhead connector and down the firewall to the transmission if stick shift.

White and violet wires if I recall