1970 Dart matching numbers 340 4 Speed

F/A Tech said
The back up lights come on but the dash"reverse" light doesn't . Where does it get power from?

4 speed car. work backwards on the diagrams.

Z in the engine compartment goes to wire B2B

So it gets the power from B2 when the switch is closed.

Since the reverse lights work, either the bulb is out or not connected or grounded.

the electrical diagram show it from bulkhead disconnect "C" is that the only connection on "C" or does it go from there to rear lights ? Thanks

Not sure what you're asking here.
Inside the car B2 goes to the boddy hanress connector.
B1 connects to X12 which comes from the fusebox.