Key on and cranking power wire?
Yeah, definitely can't use the starter relay wire for that exact reason.
I already back probed the ignition switch connector, and it's the same thing. Brown wire has power right in between run and crank and the yellow wire is the only wire that has power in the crank position.
I need to check my Scamp and see if the brown wire is doing the same thing, as I never held the key between run and crank on it. This Duster hasn't had a motor in it for years, but the Scamp was a running car before the swap, so obviously it had power to the ignition when the key was in the crank position.
I have a '71 D100 I plan on doing another magnum swap to. Maybe I'll put a battery and it and see if I can find a cranking power wire on it. It has no motor in it, so the wiring isn't connected to anything either.