You might have noticed that the fuse boxes have several empty slots. I wondered if they had power. I tried it with the ignition turned one, and most of them do. I got one of these add-a-fuse things from Amazon,


and, using a test light, I plugged it into several empty slots under the dash. Most of them lit the light. It was tricky getting the adapter plugged in, though, probably since the fuses are so deep into the box.

This means that you can add a circuit without doubling up on another circuit. The same applies to the box under the bood, so that's an easy way to add a circuit.