I have a 1989 240 sedan and the passenger side brake light is giving me trouble. Any help would be greatly appreciated!

The bulb works when the headlights are on, but will not work as a brake light. The rest of the housing unit lights up fine, as does the left housing unit (brake light included). I've tried the basics thus far, including changing the bulb, bulb casing, and light housing circuit pannel. No difference noted with the new circuit pannel. I lubricated the connectors as well, and this didn't work either. The fuse looks good with no corrosion or break.

Am I looking at a switch here, or perhaps a wiring issue? Thus far I am stuck, and since I have little electrical experience, a few pointers would greatly help.

Thanks, Scott