Back up light wiring?

You can download a complete '66 manual over at MyMopar:

http://www.mymopar.com/downloads/servicemanuals/1966_Plymouth_Service_Manual.zip

simplified diagrams from the same joint:

http://www.mymopar.com/downloads/1965/65ValiantBarracudaA.jpg

http://www.mymopar.com/downloads/1965/65ValiantBarracudaB.jpg

I can't help you too much with the physical layout. The wire comes up to the harness connector where the wire exits, going forward, from under the left sill plate, up through the left kick panel, and under the dash. From there it goes to wherever you shifter, column, or console was---you do know, right---that the reverse lamp switch in these old girls was on the shifter and not the Torqueflite?

To get the wiring to the transmission, you need to route them through the bulkhead connector, see diagram "A" top of page, 6 and 7 of the bulkhead connector. The stick cars routed the wiring from there down over the top of the bell/ transmission to the gearbox. (Same as your NSS single wire.)

You must have a late model trans if you are going to use the late model switch on the trans. The NSS on the later cars is actually TWO switches in one. The center post for the NSS does the same job as the older single terminal, the outer two switch power on/ off to the backup lamps.

So far as I know there is no way to adapt the original push--button trans to use the late 3 terminal switch.