Yeah 3 way switch mate. on | Off | on
The switch basically dictates, what triggers the relay. One side it has the 12+ve coming off the reverse light at the back, and the other side of the switch comes right off the 6 way fused block, which is a constant positive. The light feed source is also from the aux. battery.
All of this is at the back of my car, so easy access to the reverse positive.
Definitely makes it easier to reverse at night.