For some reason, the taillights on my '88 240DL sedan have reversed their roles.

When the brake is depressed (headlights off), the brake lights illuminate dimly (as if the park lights were on, and the brake was not depressed).

When the park lights/headlights are on, the brake lights illuminate brightly (as if the brake were depressed). Therefore, when the headlights are on, and the brake IS depressed, there is no change in the brightness of the brake lights, as the lights are already at full intensity.

The high-mounted brake light, independent of the other brake lights, always works properly, so at least there is a signal to other drivers that I'm stopping.

What happened, and how do I fix it?

Thanks in advance.