Steve,
If you are going to install a direct reading ammeter, I recommend you use 8 gauge wire.
Before starting, disconnect the battyery.
Using the diagram below, install the meter using the least amount of wire that will complete the connections.
Installed this way, all vehicle current, except the starter motor, will pass through the ammeter.
After installation, connect the battery and then turn on the headlights while observing the ammeter. If the needle moves to the - or negative side, the meter is indicating correctly. If it moves towards the + or positive, you will have to remove the wires from the back of the ammater and swap their positions.
(If I remember correctly, if the battery lead is connected to the + terminal, it should work OK. I don't know why I left that off the diagram.
)
NOTE: The wires shown in red are the ONLY wires that should be connected to the two large posts on the starter solenoid.