I’m assuming you’ve tried changing the bulbs? The brake/tail light is a dual filament bulb. Just cause one function works doesn’t mean the bulb is good. I would start there if you haven’t already.

You will need to find the black wire on the harness connector going to each tail lamp, learn how to test the continuity of the ground with a multimeter, then if it’s bad, either trace the wiring harness down to the local ground, or, if the ground is good - you may have a problem with the multifunction switch not communicating and telling the rear running lights to engage