I installed a Voltimeter by connecting it to a fuse terminal in the fuse box that was basically hot whenever the car was on. It worked pretty well and I think was pretty accurate. I also have an anmeter but have not installed it because it seems like it would be more difficult to rig up. good luck

Alex

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1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside