Should be able to use any standard wire it's probably 20g wire. If you're running a long distance and have any concerns of inductive voltages I'd find some twisted Bus wire.
make sure you get good mechanical and electrical connections, I use splicing bands, electrical solder, and glue filled heat shrinks on any small wiring I do. Makes for a water tight mechanical and electrical connection. Could also do the old fashioned "Western Union" splice if you're feeling nostalgic.