There are two types of lighting on our gauges, pointer lighting and dial lighting. The pointer lighting is simply accomplished via LEDs and requires 12 volts on the white wire in the red/black/white wire ribbon. The dial lighting is accomplished via an electroluminescence panel behind the dial. This panel glows when it is subject to around 200 volts AC. The inverter is a DC to AC inverter that takes 12 volts DC and creates the needed 200 volts AC to power the dial lighting. It is not necessary for gauge operation, but it is needed for gauge dial backlighting. Without it connected, the dial will not illuminate. Normally customers will wire the black wire from the inverter to a chassis ground and the white wire to the same location as the other white wire in the red/black/white ribbon.