Need help finding one particular wire...

You need IMO both the blue and the brown hooked up to the "trigger" of your accel box, one wire gives the box 12 volts signal when ignition is in the start and the other is for 12 volts when it is in Run, the coil wire comes or should come from your accel box or at least that is the way my 6al is setup.maybe your accel box is different.

x2 just deleted my regulator and ballast and ran the two wires to trigger my msd box. You also need a dedicated hot battery feed from the relay to the box.I used a one wire alternator.