Most people have gotten away from the AMP gauges and now use Volt meters. All the major automobile makers too.
I suggest you do so too...............
BUT you asked so...........
The AMP gauge should have 3 posts on it .
G) is for a ground
I) goes to hot on the starter solenoid ....use at least a 10 gauge wire
S ) goes to the key switch or the alternator
There may be another post or place for the light in the gauge.......it goes to the light switch
