Wire it so that the hot wire going to the 3 way switch connects to the "on" side of the ignition switch. One post of the ignition switch is hot and the other side is hot only when the key is turned on...assuming you have the old simple 2 position ignition switch and a stomp starter. If you have the newer style ignition switch (which has on/start/acc then I'd connect the hot wire going to the 3 way switch to the acc post so it will be on either when the ignition is on or when the key is turned to the acc side.
