I think I would look into the tailgate wiring back feeding a couple circuits.
That would make it difficult to isolate tied together issues.

It would be like working with "a clown" of a backseat mechanic, a gremlin, messing with you!

A power cross up back there turns on the defrost relay light. I have seen that happen with brake lights. It was random, but ever time it did it, it was after I used the brakes.

The headlight switch is new to me and the possible cross up in the taillights. Check bulb filaments to be in the right socket locations. An Owner's fingers might be the creative gremlin!

The possible powering up location might power going the headlights while the running lights circuit is on.

The staying on is a part of the defrost time out. Do the headlights go off after ten minutes or so, even if the headlight switch is on or off?

Good luck

Phil