[dmsfrr]

Does the voltmeter have two terminals, not connected to the housing that holds the gauges or anything else?
If so...
and the vehicle is Negative ground, the Battery / Plus terminal of the voltmeter would connect to the Accessory terminal of the Ignition switch, and the Negative voltmeter terminal to a good Ground.

If the vehicle is Positive ground, the Negative terminal of the voltmeter would connect to the Accessory terminal of the Ignition switch, and the Plus terminal of the voltmeter connects to a good Ground.

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