You want to use a switched 12 volt source for a voltage meter, so no, don't use the ammeter wires. You can jump the wire at the input of your dash volt limiter for the source. Depending on your car, that might be at the fuel gauge.
If you're leaving the ammeter in the car, you can tie the old wires by putting them on the same post of the meter. Better solutions are bolting them together, or cutting the terminals, splicing and soldering them. Insulating the connection with electrical tape, shrink tube, or better yet, both is a solid way to do it.