A voltmeter is added to the accessory bus. On '65-'67 Mustangs this is a screw in the center of the ignition switch. On '68 and later, I'd have to refer to schematic wiring for the model year.
It is not added to the IVR bus, which is the instrument regulator bus. This would cause a modern voltmeter to jump all around and never give a constant reading like you describe. The fuel guage and temperature guage are on the IVR bus on an early Mustang... you can't use this for your voltmeter.
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