The 65-66 ammeter works by measuring voltage from one end of a wire to the other.  It does not measure charging current directly.  I am no electrical wizard, but the gauges just suck. They barely move.
So, I figured out a way to mount and wire a REAL Stewart Warner AMMETER in the original 65-66 dash.
First, take a hammer to a S/W ammeter.  No, wait, use something more subtle, but disassemble the gauge to retrieve only the working parts of the gauge and the needle and two studs.  Don't worry if you are not electrically technical, this is real easy.  Then, remove the 65/66 gauge and disassemble it so you only have the actual sheet metal face of the gauge remaining.

Now, the 65/66 gauge pivots at the top, and the S/W gauge pivots at the bottom.  This works in your favor, as you can use some rtv to glue the 65/66 face to the S/W mechanism.  You would think it would make the gauge read backwards, but simply wire it backwards and it will register correctly.  Then, mount the 65/66 face with the gauge attached to the 65/66 housing, making sure the two studs do not short to the body of the gauge cluster.  Then, just wire the gauge per the S/W instructions and you will have a full function ammeter in your car.  Be sure to paint the gauge's needle with some day-glo orange paint to match the originals.  Actually, your originals WERE day-glo orange when new, so repaint them all so they look better.  This is a really easy fix for a 50 year old problem.

And, if you really want to know what your charging system is REALLY doing, mount a simple voltmeter in the glove box.  I have found a voltage gauge to be much more usable.


This pic shows the completed part to be installed in the 65/66 Gauge cluster.  Note the face of the S/W gauge peeking out (upside down) from the Mustang gauge's original AMP gauge face. 



This is the back side view after it is all assembled.  Be sure the two studs do not short on the cluster housing when installed.  This wires in differently than the original ammeter wiring.  You can actually run CURRENT across this gauge.  You may need to use some RTV to mount the gauge in the housing.  Once installed, it looks stock.