For a amp meter to function properly all power needs to go through it. (Charge and discharge)
You will find that the usual wiring has the altinator out put going to the battery from the battery there is a lead that goes to the starter solenoid and from there is where the main feed goes to the fuse box.
Wiring direction - disconnect both wires from the starter solenoid and disconnect the charge wire that goes to the battery.
Reconnect the wire from the battery to the starter solenoid and run a new wire from this connection to the amp meter -ve. Run a wire from the positive of the amp meter and connect it to the main feed (which was disconnected from the start solenoid and tee in the alternator wire. You might find it easier to extend the main feed and alt wire and connect both to the amp meter.
If gauge works in reverse change wires on rear - you can't damage the gauge but use a sufficiently heavy gauge wire. - Nothig worse then the smell of melting insulation