The reverse switch is usually on the gearbox or the selector mechanism.
What i would look for is the wire going to the rear which feeds the reverse lights and tap into that. The camera will only need a signal, not a large current, however, you should still make sure to fit a suitable fuse.
Based on the comment:
I would just find the color of the wire at the rear light and cut into the wiring loom at a suitable point to get the same color wire - it will have a base color and a trace color. And, if you are color blind get help - one of my friends made such a mess of his motorbike wiring... He was a fantastic metal worker, but couldn't do colors - we had an agreement - very handy for beating out dents : wires I find easy...