Unlike the OEM Veglia gauges, the case of the Hall voltmeter isn't grounded, you need to attach a ground to the post marked negative and switched power to the positive post.
If you use the bulb that comes with it, you need to connect a ground to one of the bulb wires and use the stock amp meter's bulb positive on the other side so it will dim with the other gauges.
I ran a "fresh" switched 12V wire from my fuse panel so I can't answer where the best place to get switched 12v back there is.
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