I have a 2006 Tacoma. I am having a problem with the rear break light on one side that I can not figure out.

With the headlights out, if I step on the break, both break lights (left and right) go on. When I turn on the headlights, both tail lights go on. With the headlights on, if I step on the break, the break light on the right side goes on but the break light on the left side does not AND the taillight on the left side goes out (no lights on the left side).

I have replaced the bulb but there was no change. What’s going on??

Thanks for the help.