Car is a 73 duster with a 74 harness.73 harness was really cut up...no wiring on the right side was being used..it was all jury rigged ..large wires under dash were cut and spliced with wire that looked like lite plug wire and wire nuts...tx .j
Anything you do to this 74 harness will be somewhat similar to the 73 harness you described.
I could convert your stock amp gauge to a volt gauge for you but...
How the two instruments are wired into the system/harness is completely different, which will require some hacking.
To add a aftermarket volt meter under the dash is a lot less radical.
How you go about taking the amp gauge out of the system is up to you. Some choose to simply stack both of the ring terminal on one post of the amp gauge. Others cut the ring terminals off and solder, crimp connect, etc..
Others buy a #10 X 1/2" bolt and use the stock toothed washers and nuts on that, with ample electric tape wrap/insulation. That's probably the least invasive most effective of these. The connection is totally separated from the amp gauge yet can be put right back like OEM at any time.
I'm sure there are more methods to the madness .
