First find out if it's a true ammeter - wired in series. True ones have two (heavy) studs on the back and are spliced into the output wire of the gen/alt. Most vehicles from the 70's up don't use this one because your sending unfused power in and back out of the passenger compartment and it requires a heavy wire - at least as heavy as the alt output wire. Heavy wire costs money and the car makers don't want to spend a nickel. I'm sure there are other reasons, such as voltage drop, fire hazard, new technologies, voltmeters tell you more, etc.
The other unit is a voltmeter that lookes like an ammeter that measures voltage drop and is expressed as amperage - and is wired in parrallel. It usually is wired (16-18 ga wire) at the output terminal of the alt and at the accessory side of the igition switch - shuts off when the key is out. The terminal ends on this unit are small, usually spade connectors (but not always). Do Not wire this one in series.
Why would want to install an ammeter instead of a voltmeter? The voltmeter just wires into an accessory feed (power on with ignition) and a good ground - all of which is inside the car.
Pete