Where To Hook Voltage Meter?

To the fuse panel on any switched line.

I'd add one comment. I like a voltmeter to reference charge/discharge at the battery.

You might check your source (whatever switched circuit you choose, as JR mentions) for voltage drop. Some of these cars are getting old and some circuits are dropping some volts. This should not be a real issue but it's something that I consider and do (having some older cars around necessitates this).

I'd choose a circuit that is reading approx the same as the battery.

Good luck.