Yep you only need three of the six tabs on that switch, use three that are in a row on the side. Connect the switch's center tab to the light itself. Connect one of the other two to the Reverse light connection, connect the remaining switch tab to a continuous source of +12v. Make sure the switch has a center-off position. In one switch position, the light will come on manually. In the other position, it will come on when you shift into Reverse. It will be off with the switch in its center position.
The fuse box would be a good source of power but not if you are just planning to stick a wire into a fuse slot in the front. Probably easier just to run a wire directly to the battery, just make sure to have an in-line fuse in the wire to protect the circuit. A 10 amp fuse will be plenty.