[hatzie]

You're moving the ignition START wire that runs from the column to the neutral safety or clutch interlock switch to your add on button.... on a 1979 this wire is yellow 3 YEL-5 in the wiring diagram (Marked SOL on the switch). The other side of the button needs to be hooked to an ignition switched 12v source.

I'd slip the Packard 56 terminal out of the safety switch and tape over it. Then add a new wire and Packard 56 terminal that reaches the button. That way you can reverse it without too much trouble.

I remember reading about a fellow that used a magnetic switch mounted to the back of the knee panel to interrupt the yellow wire. The start position on the ignition switch start position didn't work without a magnet stuck on the knee panel in the right location.