Tail Light Wiring Help Needed

Thanks for the responses. I was wondering if it might be a ground issue myself, but I traced the brown and black wires all the way up to where they connect behind the drivers side kick panel and I am getting power to both wires up there when the headlights are on. Therefore, it seems to me that it probably is not a grounding issue....although perhaps I am wrong about that? Electronics is not my strong suit. Perhaps I will try a dedicated ground wire back on the bulb socket as TrailBeast suggested and see if that makes a difference. Thanks again for the suggestions!