I'd imagine it would be part of the gauge wiring harness, however if you picked up one of those gauges, it looks like you would be able to tap into the signal wires from the gas, oil, and overtemp sensors, and it probably has a built in voltmeter that will light up when voltage drops below spec. If I were to wire my ski, I would just pick up some 4 wire trailer wire, or network cable (wires are thinner though) and just heatshrink and solder all the connections.
If I ever pick up an older ski with a gauge pod, I would love to install an actual engine temperature gauge (that way i could see the gauge rise and back off the throttle, before its too late) I've got a parts skidoo kicking around. Just going to extend the wires. Now to find another ski.. lol