adding a NSS

Set the switch up, mechanically, like the brake switch, that is. So it comes off when the pedal is pressed. The brake switch is actually a positive side switch, so both wires are a positive/ positive. All it does in the brake circuit is act as a connect/disconnect for the completed positive wire.

This is the same thing, only it is a negative side switch and acts as a ground connect/disconnect for the NSS terminal on the starter relay.

Not to be confused with a shorting switch, which grounds out a positive side, like a kill switch on a lawnmower magneto. What I'm saying is that you don't want to copy the brake circuit and connect any terminal to a positive feed. You just copy the brake switch's mechanical switching function on the pedal and mount the switch itself in the same functional manor.