Which ignition wire?

There is no "one wire."

You need to understand how Mopar igntiion switches work, which of course are actually SEVERAL switches in one container.

For ignition/ starting there are three

1--- Traditional yellow, goes to starter relay, and in conjunction with neutral safety switch and starter relay, cranks the engine

2--- Traditional dark blue, "ignition run" or IGN 1, goes from switch, through bulkhead, to one side of the ballast resistor, and also feeds the regulator IGN terminal, and on 70/ later cars, also feeds the blue alternator field, electric choke if used, idle solenoid if used, etc, maybe a couple of smog doo dads.

THE ABOVE WIRE is hot ONLY in run and GOES DEAD in "start."

3--- Traditional brown, the coil resistor bypass, or "IGN 2" goes from switch, through the firewall, to the coil+ side of the ballast resistor. HOT ONLY during crank.

The easy way to do this is to bypass the coil resistor, which will result in the blue and brown being tied together. Then take your original coil + wire and hook it to the red trigger wire on your MSD

To conclude, the brown bypass is hot in "crank" and the blue "run" wire is hot in "run"