Typically there are 4 wires on the auto trans. Inhibitor switch.
Black, GROUND
Black/Yellow tracer, START signal from Ignition Switch
Black/Yellow tracer [possibly], START signal to START relay
Black/Red tracer, REVERSE signal to Reverse relay.
These are the wires used on a 510 so use them as a guide only

The auto trans Inhibitor Switch usually outputs an Earth or Ground signal when the selector is in "R".
Have a look for a BR [Black/Red tracer] wire on the Inhibitor Switch body harness plug.
Try earthing that wire with the ignition switch ON and see if the Reverse Lamps light up.
If they do then you connect that wire to the manual trans Reverse Switch.
Then connect a ground wire to the other terminal of the Reverse switch.
You also have a Start relay with the Auto. trans that must be energised or the starter motor won't get power.
That will need two wires bridged together to take the place of the Inhibitor switch.
Likely to be a BY [Black/Yellow tracer] wire and the wire that goes to the Start relay.