Well, I thought I was dancin'...

Your........brake.........light.........switch........is........stuck...........on.

With the sockets hanging out of the body, the current comes through the stop / turn filaments and "is looking" for a ground return. With the HL switch off, you have the tail lights UN powered. I believe they find a ground up through the park light filaments to ground.

Only other possibility I can think of is if the harness contains a trunk lamp circuit and the harness is pinched or "welded" causing a cross connection in the harness.