Jones, my tranny didn't have a switch, neither did my shifter, (pics are in my build).
I have a bank of switches that has 12 volts going to them when the key is on.
The switch for the backup light sends 12 volts to the LED light I mounted in the back.
In your case with a switch in the tranny you need to basically do the same, 12v to one side of the switch, the other switch wire will go to the + side of the backup light (if your using an LED light), the other light wire will go to chassis ground.
Tim, nice pics.