Key on and cranking power wire?

Doing my second A-body magnum EFI swap on a friend's '72 Duster. When I did the swap in my Scamp a couple years ago, I couldn't find a wire that had power with the key in the run and the crank position, so I had to toggle switch the power wire for the computer. Thought maybe it was a problem with the car.

Now on this Duster I'm having the same issue. What are you guys using to power the computer?