i've looked through all the V300B material i have on this and it does not show a voltmeter/amp guage in any of the diagrams or harness schematics. Which makes me wonder if someone added this guage at some point. The only additional guages it shows are for air pressure or oil pressure.
But generally with voltmeters there are usually only 2 wires that connect to it so it can measure voltage in the system off the ignition circuit. If it is setup to be used as an amp guage then the wires would be large guage like 10 or 12 but still would only be 2 wires.

This lone wire u say pegs out the meter when you connect it could be anything but obviously does not belong connected to that guage if it pegs the guage when it is connected, that is just shorting out the guage circuit when it's connected and will burn out the guage if you do that for too long. So dont do it :o)

I'll keep looking but so far on this age of truck my resources are limited, this was before cat was very proficient in making their material available on electronic format, most of what you will find would be in a paperback manual if you can find one ;o)