The left brake light on my 2004 escalade base awd is out.

I have checked the bulb and fuses and both are working fine. I figure it can't be the switch since my right light goes on, and it's not the bulb or fuse because it lights up as a running light.

I figure that the malfunction must be coming from the junction where the wiring for the rear lights meet, which is behind the rear passenger side tire, above the exhaust.

I had this problem before, and the mechanic stated that it might be all the junk being throw up from the tire, clogging and dirtying up the connections. Why Cadillac would choose such a horrible place to put this junction idk. He cleaned it out and it seemed to return to functionality. I have since tried this to fix this current issue but to no avail.

My question is whether or not the mechanics explanation seems plausible, and also, what the junction piece is actually called and where i can get a replacement part. Also, if my brake issue is being caused by something else and that my conclusion is wrong, please clarify.