With electric forklifts and electric pallet jacks, EVERYTHING goes back to battery negative. You have ONE common negative and one common positive connection. All negative wires are generally numbered "13". It is a much better design than this crappy grounding to the chassie crap they do on vehicles. Anytime I add an accessory, it gets a feed from the switched lead or directly to the battery and always directly to battery negative. I have to do it when adding a horn, back up alarm, headlights or strobe light to an electric forklift,so I treat the engine powered things the same way.