[Cdncarnut]

If you are going to mount a switch, you can get your power from anywhere.

The issue with me using the clutch interlock was that the power was off, with the clutch pedal depressed.
That is why I had to use a relay to provide 12 V to the 2-step to engage it.

The problem with using a switch, on the shifter, or something similar is that you have to be coordinated.
When you launch you will have to let go of the brake and the button.
Unless your ideal is to use the brake switch and button on the shifter in series.