[Humperhill]

That does sound a bit tricky. You say the brake lights work properly if the tail lights and tail lights are on, but do not work at all if headlights and tail lights are not turned on. Sounds like the power going to your brake light switch might be coming from your headlight switch and not off the battery. It should be wired so that as soon as the key is in a run position you have power to the brake light. You might want to try to follow your wire form the brake light switch back to its source. Maybe at one time the ignition switch was replaced and not wired back correctly. I am no expert when it comes to electrical, but this is what it seems to me.