Burnt some Spaghetti!

The fact that someone added the gages under the dash which essentially replicate the gages in the rally instrument panel suggests a bunch of alterations.

Check the under dash gages to which ones are connected.
If the ammeter is connected properly, the R terminal will have a wire from the battery positive. The other terminal will have a wire from the alternator output - possible via the main splice.
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We can look at Hamtramck Historical to learn the options available at the dealer.

If the car had rear defroster grid, it was supposed to get the 60 or 65 amp alternator and modified wiring. The clue will be heavy wires through a grommet in the firewall.