If the guage you have added is a Voltage guage then tie the two wires from the old Amp guage together and tie them to the + side of Volt guage.
Tie the - side of the volt guage to ground. BE SURE the "Volt" guage is really a volt guage.
Sorry no you don't want that either, "but close". The two old ammeter wires are always "hot." You want the voltmeter hooked to a switched source, IE coming from the ignition switch
