Are you cutting into the cable because you want to get rid of the original coil arrangement as well as install the Pertronix? If so, the wire within the armour is +12 v supply to the coil. It will go to the +12v on the new coil. The red wire coming from the Pertronix module also connects to the +12 v terminal of the coil. The black wire from the Pertronix module goes to the negative terminal of the ignition coil. Whatever wire that you currently have that connects from the coil to the distributor points should end up in the spare parts box along with the old coil, ignition points and condenser.

You may or may not be aware that if all you want to do is install a Pertronix module, you don't need to cut the armoured cable or get rid of your old coil. There is a thing called the diode kit that IPD sells which allows you to install the module without messing with the coil and the armoured cable