bartamos,

The single wire from your alternator would go to the - side of the ammeter along with all the loads...with the exception of the horn, but including the keyswitch.

The + side would feed the battery, and the horn. This is the basic wiring as I see it.

Many folks think having a meter in the dash that has the ability to pass lots of current is a hazard. A voltmeter works fine to monitor the battery and charging system and doesn't pass current. This is probably why your harness manufacturer supplied wires for a voltmeter and not a ammeter.

The alternator makes the 'juice' which goes through the ammeter on its way to the battery. Battery side is + and alternator side is -...simple as falling off a log!

Stuart