Red/green is a ground. White/green is a modulated ground. If you were able to dim the lights with the red/green supplying the ground then the ground line running to the dimmer is bad or that portion of the potential divider is bad (hence a bad dimmer). Test the white/black wire for continuity with ground. As I've said before, jumpering the white/green with the white/black wire will make the gauges light up without the dimmer present... it's the easiest way to diagnose a bad dimmer.
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