An ammeter would go in series between the alternator and the battery (typically). If you want to monitor the current for a specific item, you can just wire it up for that (say an amp, ignition system, etc.). Heavy gauge wire has to be used, and you don't usually see ammeters anymore. A voltmeter on the other had is what 99.9999% of cars have now. It just hooks up to an ignition switched 12v source (same circuit as the gauges in a factory cluster). Unless you want to see how much current is running through the system (not voltage), imo it's somewhat pointless.
