Ralph, I'm sorry. Did you not get my email? When using the 65 GT wiring harness, ammeter or not, those two large gauge wires plug into each other. That's the main power. Email me at firstname.lastname@example.org
if you have any further questions.
p.s. Here is a picture of the back of a 65 GT cluster showing how the large wire loops through the ammeter. Just connect these two large gauge wires. They won't fit any other wiring in the harness.
All my best.
Lance A. Morgan
65-73 Mustang electrical repair
webmaster, Mustang Grabber Registry