Battery is good, so is alternator. Now it won't start or show any electrical activity. What could the problem be?

What is your question?

The battery is new and has been tested, the alternator is new. The Mustang will not start, headlights won't turn on or power locks won't work. I changed the postitive terminal, and still not showing any signs of electrical activity. Could the issue be with a fuse or it there something I'm missing?