cut wire ?

Your car probably has the heavy duty ammeter wiring, sometimes called fleet/ police/ taxi wiring. The ammeter is routed through separate grommets in the firewall with heavier wire, and the original red ammeter feed wire is taped off. I cannot tell from your photo for certain, but the two separate red/ black ammeter wires (two black, 1 red, and the cut wire) make it seem so.

A thread 'er two

http://www.forabodiesonly.com/mopar/showthread.php?t=184696

If you look at the rather convoluted diagram from the 74 manual, it shows "both," but does not make really clear how this goes

Find the ammeter, and below it "grommet" near the center of the page. This grommet is where the heavy fleet wiring is routed through the firewall. Those two large no10 wires are two shown in your photo

The original wiring, R6C--12BK is what feeds power to the interior of the car and is still used

The wire which is now unused is the one at the right side of the ammeter, going up, no A10-12R. It used to go out through the troublesome bulkhead connector, which the heavier grommet adds better integrity