Someone here posted this on an ammeter to voltmeter swap not too long ago:
You can use the same spot in the carrier - but do not use the connections to the PC board. The ammeter is wired in series with the electrical load - a volt meter must be wired in parallel. It would not only not work, it would most likely immediately smoke your harness.
With the instrument cluster open, release the ammeter from it's posts. When it is out, remove the copper spring clips that connected the ammeter screws to the PC board - discard them.
That is all there is to eliminating the ammeter.
To feed the voltmeter through the PC board would require some involved modifications to the wiring harness. That was the reason for chucking the spring connectors - that and because it would prevent the voltmeter from seeing unregulated voltage.
But the voltmeter can read the vehicle's voltage from any point in the system. Note: the hot wire should be ignition switched (i.e. fed from a circuit that is only live when the key is in the run position).
Make 2 fairly long leads - maybe 3 feet long (best if one is red and one is black). They will run from the voltmeter, out of the back of the carrier and over to the fuseblock (red wire) and the common ground bus (black).
Use ring terminals at the voltmeter ends - and attach them to the posts with 10-24 nuts. Drill a hole, or fish the leads out of the carrier, and run them as decribed above.
Remount the voltmeter using the same posts as the ammeter and close the cluster up.
Use an in-line fuse on the hot red wire if you go straight into the fuse block (use the "I" bank). You can also find a convenient existing fused circuit to tap into (radio hot wire, clock, etc.) anything that is only powered up in the "RUN" position.
The black wire will go to any place that provides a good path to ground - as mentioned the common ground bus is good.
