Despite the information given here, the Pertronix II and III will not function as wired by ford with the tach. Not an assuption, but fact. It MUST have a 12V power source to the module. Even the Pertronx I require it, despite them running... For a while. I have a Shelby with a factory tach and have been through this.
The diagram Pertronix supplies is NOT Mustang specific and is wrong. It's why I keep talking to people in messages and giving them instructions to walk them through. The II and III will damage a factory tach without proper wiring.
Many wind up getting talked into a three-wire tach movement when they don't need it because of this.