I have mental plans to do the same, the on-off switch would be idea to get a moronic taligater off my bumper
Run power from your source of 12V power sources switched/constant would be your choice.
Run this through a relay triggered by factory reverse lights with rectifier diode between the relay and the tap in the reverse light circuit. Run a switch providing 12V small wire to the same trigger point on the relay.
The diode will prevent you from powering the stock reverse lights when you flip your switch. Yet will allow you to use the auxillary lights when not in reverse.
Attached is my pitiful attempt at a wiring diagram in paint.