A wiring harness with an ammeter only or a volt meter only are 2 different animals.
If you had one that wired for an ammeter and you wanted a voltmeter, that's very easy, you just have to attach any hot wire to the meter and it will display the voltage in the system..
However if the harness is wired for a voltmeter only, it may be virtually impossible to adapt to an ammeter. That's because the feed wire from the alternator may be routed directly to the Main terminal at the starter and never pass through the cab.
For an ammeter to read correctly, all of the power for the vehicle has to pass through it. One very heavy wire from the battery (usually connected to the battery terminal of the starter) runs up the harness to the ammeter. Then wires that feed the rest of the electrical system run off the other ammeter terminal (ignition switch, fuse box, etc). The ammeter than reads the flow of current to all parts of the system.
The voltmeter only harness most likely runs the power feeds in a way that cutting open the harness to splice in an ammeter feed would not be practical.
You could remove the ammeter from the gauge cluster and replace it with a volt meter and have it look original. I've done it and there is a good tech note on the procedure.
