There may be extra wires that have connectors on them (I believe they are solid orange) under dash for things like shift indicator lights. You could use these without any spicing. I'd get a wiring diagram for your car just to be sure.
Extra connectors on the factory harness? Hmm...this might work for the voltage gauge perfectly.
I would prefer to have the gauge lights on the switch with the dash lights so they would come in together rather than running all the time. Then again, my dash lights are increasingly unreliable so I need to look at that too.