It doesn't show on the schematics, but all the console gauges are 'daisy-chained' together for their grounds. Sounds like the whole console ground somehow got loose and since the console is fiberglas, everything attached goes non-conducting. There's also a LOT of amperage going thru the ammeter so it would pay off if you removed the panel and started looking for melted insulation or evil smells in the gauge wire bundle. Failing that dig out the VOM and start looking for lost electrons.
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