Finding switched 12 volt source

No the yellow start wire is separate

You do not HAVE to strip out the harness wires. You can simply jumper all ballast resistor wires together, take the original coil+ and use that to power the new system. If you have MSD hook that wire to the MSD "small red"

What this will create, if you unplug the old Mopar box, is the original coil NEG wire is dead, the pickup wires are dead, and there will be power on the Mopar box connector, so "safe" that off

You don't even need to hack up the ballast connectors. You can get male-to male flag connectors and insert them into the original connectors. If you want to maintain a stock appearance, you might be able to "hack up" a 4 pin resistor and jumper all 4 terminals together. ---and actually, it's only two. One of the 4 goes to the old box
Original wiring:
Two of the ballast terminals at one end are already jumpered by the harness. Most harnesses you can look and see those at one end of the ballast

One remaining goes to the old box connector

Last one goes to coil+