Postscript -- that is a very good link. I called Pertronix sometime back while troubleshooting a problem I was having. Remember that the spark comes from the coil, the Pertronix unit is merely a switch with a set dwell (in the Pertronix 1). And as was said in that link either works or doesn't. If I recall correctly, it will work down to about 9V but I'm not sure about that. Any degradation in spark is most likely something to do with the coil, wires, timing, plugs, cap, distributor rotor or plugs. For example, I once had a Sunbeam Tiger and the coil would only misfire when the engine was hot and a break in a winding would open up due to temperature. Cold it was okay. To directly answer your question, the red and the resistor (ignitinon) wires go to the (+) side of the coil, black to the negative side. Be sure you set the correct clearance between the rotor and pickup and you should be good to go. I have an old Pertronix unit and that clearance is .030. I use a piece of plastic to set it. They say that metal is not good. I don't know what the clearance is on the new units; I understand that they have been updated.