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AM amp gauges hook in between the alt./gen. output and the battery side of the starter solenoid. You need to use HEAVY gauge wire and make sure the ammeter has a higher rating than your alt/gen. I used 10 gauge wire for my instal with a 65 amp alt. and have had no problems. Keep your wires as short as possible and use good connectors at each end. DO NOT splice the wires. It would also be a good idea to add a fusible link in the circuit to protect the wiring.
You hook to the same large post your positive battery cable runs too. You must remove the wire from the solenoid that now comes from the alt/gen. That wire must hook to new wire you install that runs to amp gauge. Then run new wire from other side of amp gauge to solenoid where oe wire was.
I recommend against amp meters for cars in general. They are potentially dangerous and don't really give reliable information.
Dangerous because to be accurate, all the current going into or out of the battery must pass through it. Since the meter is in the cab, a heavy guage wire must be run up into the dash and back out to the battery. If this wire gets frayed it will cause a short that could start a fire.
Unreliable: If you connect it where suggested (which btw is where the factory puts it) you only get an idea if current is going into the battery during the charge cycle. After the battery is charged the current will drop to near zero. In order to get an accurate picture of the charge or discharge state of the electrical system the alternator and general electrical hookup to the rest of the truck should go to an insulated post that is not the starter solenoid. Then a wire should go from this post, throught the ammeter and back to the battery side of the solenoid. This will then tell you if current is flowing into the battery (charging) or out of the battery (discharging). The current to the starter would not go through the ammeter so it will be protected from that high surge.
A much simpler, safer and more accurate guage would be a volt meter. Connect between a 12V source and the chassis of the vehicle.