You'd just run a small single wire from the amp to your gauge so that it can measure voltage. Digital voltmeters have high impedance inputs so they draw very little power from the source they are measuring, hence voltage loss/drop from using a small wire will be extremely insignificant. My voltmeter gets it's power and measures from the cigarette lighter supply. Compared to my Fluke multimeter accross te battery terminals, it's readings are spot on!!