How do you wire the Discharge Gates switch on a semi belly dump trailer?

Well, you'd have to know how the trailer is wired, first. There'll be two plugs on the trailer... one, for the regular seven pin pigtail, and the other will be an auxiliary through which your power for the gate controls, hazard recognition lights (if so equipped), etc. will run through.... this one will usually be smaller, and may only be four or five pins, one of those being a ground. You need to figure out which prongs on that connector go to the gate discharge controls. A 12v battery and some jumper wires should help you narrow it down. You connect one end of the wire to your power source, then touch the other end to the prongs, and see what it does. If one pin is larger than the others, that's typically your ground.