It looks like half the wiring diagram is in the photographs. It looks like he drilled a hole in the rear of the light housing and ran in two wires. The ends of the wires were soldered to the positive contact point for one battery and the negative of the other (it might not matter which). The hole would be sealed with silicone sealer from the hardware store. The other ends of the wires could be attached to some sort of connector to make each light independently removeable. I can only imagine the rest.