Voltmeters are usually just wired to any convenient point in the electrical system that is ON when the key is on. Typically you would run a positive lead directly to some unused spot on the fuse box. It really wouldn't matter, and actually would be more informative for troubleshooting, if you wired it to something that's ON all the time, such as the parking light feed circuit or the hazard lights circuit. That way you could see what happens when you start the car.
The voltmeter is monitoring system voltage. Essentially this is the same as the voltage across the battery or the output of the alternator. Most wires in a car are short enough that there is almost no voltage drop in the system, so those two points *ought* to be the same. Reality sets in in the guise of lousy, oxidized terminals and nuts and bolts. 15 years outside will do that to you. So the output of the alternator *could* be a bit higher than what you measure elsewhere. Anyway...
An ammeter (amp meter) indicates the real output of the alternator, and would tell you whether the alternator is actually charging the battery or whether the car is drawing more power than the alternator can deliver.
The normal place for an ammeter is in the wire going from the battery to the car's main accessory wire. This can be a smaller wire right at the battery cable clamp or a wire leading away from the BIG starter terminal, depending on the car. ALL the car's power, except for starter motor current (>150AMPS!) goes through the ammeter. This means that fairly large wiring has to come right into the dashboard- a pretty long run and potentially a pain to install. Conversely a voltmeter is simply wired from any positive point to ground- across the line voltage, if you will, and essentially NO current goes through it, so tiny wires are perfectly sufficient.
Hope this answers a couple of your questions.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 805K total