Ammeter to Voltmeter...who does it?

I just wrote a long reply only to have it crap out when my picture had a bad extension so excuse me if this is too brief...

Amp and volt meter needle movements are the same. Just a little coil that forms an electromagnet and moves a needle.

Connected as a voltmeter, there's a high ohm resistor in series that minimizes elec flow through the sensitive movement.

Connected as an ampmeter, there's a low ohm resister in parallel that shunts most of the electrical flow away from the needle movement.

Get a junk Chev or Ford voltmeter, cut it open, see if you can remove the series resistor or at least measure its value before going to Radio Shack. Remove the shunt resistor from the Mopar amp meter and install the correct series resistor and the sex change is complete.

This sketch shows the difference. You wouldn't connect it quite like this in the car because you wouldn't want the volt meter energized with the key off. I would suggest connecting the two heavy ga wires that went to the old Mopar amp guage together, connect a light ga wire to this same joint, connect the wire to the hot side of the newly configured guage and a second wire from the other side of the guage to ground. It is now a volt meter.