Sounds like it is working which is very unusual for a '66 ammeter. I have a '65 GT which includes an ammeter, but of a different design. At idle, the needle rests at or around the middle of the gauge, depending on the state of charge of the battery and the power draw at idle (i.e., lights or fan on, etc). Again, depending on the state of charge, while driving my ammeter moves very far to the right which indicates that the system is charging (it is measuring current, not voltage). However, due to the way the voltage regulator is designed, when the system is fully charged, the needle will usually start to bounce which simply indicates that the current is switching on and off rapidly as a result of reaching a particular design voltage.
If you don't like the behavior, you can replace the ammeter with a voltmeter which will give you a much more stable needle and slightly different information regarding the state of the charging system.
1965 A Code GT Fastback