Need to find switched 12v wire that's hot while cranking & with key in ON position

The picture shows the system I'm using. But, I'm running an MP distributor, instead of an Accel unit. Otherwise there is no difference.
I've got it wired exactly like the instructions say, except for the switched wire. As you can see, it's a really simple set up.
I just can't find a suitable point where to connect it.