This is a system which seems to be a prolific source of confusion and despair for people, but it is actually quite simple, once you understand it. For all practical purposes, the only thing that your battery is supposed to do is supply power to start your car. After the car starts, all of the lights and gauges, and the motor itself, is powered by the alternator. The alternator also makes sure that the battery is fully charged for the next time you want to start your car. The alternator generates power and sends that out into the cars electrical system. This system, like all other electrical systems, can't just push power out into the void. The power also has to have a way to get back to where it started from, or, a ground. That's why they call them electrical circuits. The power has to take the entire tour. You have a black wire with a yellow stripe coming off of the alternator post marked ' Bat' that goes to the hot side of the starter solenoid. This is the same post that your positive battery cable goes to from the battery. You have a black wire with a red stripe that is attached to the post on the back of the alternator marked 'Grd'. This is, that's right, the ground. This is basically one wire with three ends. One end is attached to the alternator, one end is attached to the engine block on the same bolt that the negative battery cable is attached to, and the third end goes through the headlight harness and is attached to the radiator core support with one of the screws that holds the voltage regulator onto the car. You have a white wire that is attached to the back of the alternator on the post marked 'Fld. This wire goes over to the voltage regulator and is going into the terminal marked F on the side of the voltage regulator. It should be the terminal on the bottom with the voltage regulator mounted on the car with the plug thingy on the driver's side. There will be a yellow wire that goes from the voltage regulator terminal marked 'A' over to the hot side of the starter solenoid and that's all that there is for the actual functioning of the alternator itself. The alternator generates nearly 15V, it goes to the voltage regulator and gets knocked down to 12V, comes back and goes into the electrical system of the car. This part of the system is the same, whether you have an alternator light or an ammeter gauge. The differences between the two systems are all in the way that the charge indicator tells you what's happening with the system.
The charge indicator light works like this. There is a white wire with a black stripe attached to the post on the back of the alternator marked 'Sta'. That wire goes to the voltage regulator and then winds it's way to the charge indicator light and miraculously turns green with a red stripe along the way. There is another wire that is black with a green stripe that jumps off of the red wire with a green stripe that comes from the ignition switch and then goes to the charge indicator light. The light will have two wires coming out of the back of it, one black with a green stripe from the ignition switch and one green with a red stripe coming from the voltage regulator. The two pictures of the back of an alternator show how the wires are supposed to be attached to the back of it. The one with three wires attached to it, without one attached to the stator post is for a car with an ammeter gauge, and, the one with four wires attached to it is four a car with the charge indicator light.
The ammeter gauge works in one of two ways. On a 66 model car, it will have a red wire that goes out to the hot side of the starter solenoid, and a yellow wire that splices into the black wire with a yellow stripe that also goes to the hot side of the starter solenoid.
On a 65 model car that came with either the GT package ( performance/image option) or the pony interior ( Interior Decor Group) the ammeter gauge doesn't actually have any wires attached to it. It has that big black wire with the yellow stripe passing through a metal loop which is attached to the back of the ammeter gauge without actually touching anything. If I didn't have any understanding at all about automotive electrical systems and was asked to pick which one of these two systems normally doesn't work, the 65 system is the one that I would pick, since there aren't any wires attached to the gauge. But, I would be wrong. The 65 ammeter gauges almost always still work forty plus years later, and the 66 ammeter gauges, more often than not, didn't work brand new. There is nothing strange about your 66 ammeter gauge not working. It's nothing personal. They treat everyone like that.