The directions on the package tell you to go directly to the positive post on your coil with the red positive wire. That will work, but if you somehow drop below 7.5 volts at the coil, your pertronix will stop working. To get a constant 12 volts to the pertronix, here is how to hook it up to a switched 12 Volt source.
http://va-mustang.com/articles.php?articleId=3

 

Yes, Do that, and don't agrue with Soaring, I did enough of that when i installed mine.