Help with White Face Gauge Wiring

I recently purchased a set of Stack Racing white face gauge inserts from American Muscle. The directions are not clear about how to wire these. The wiring harness has a 12v power inverter with 3 wires coming out of it. One wire is obviously the connecting wire for the gauges. Another is a dimmer knob and the third I have no clue. Maybe the third also connects to the dimmer knob, but came apart during shipping. Regardless, I want to wire these gauges to my headlight knob. Do I need the 12v black box? Do I need the dimmer knob if I plan to use my headlight knob to dim/brighten? Do I cut the wire off the black box that connects to the gauge inserts and wire the red wire to my headlight knob and the black to a ground lug?

Any help would be appreciated!

Thanks,
Mike