From what I can tell, it's the plug that the whole rear harness attaches to, but I'm just looking at a B&W drawing.
You have three choices, make it look OEM, make it look half-assed or make it better then OEM. I'd probably run some better gauge wire from the stereo. 18 gauge wire is tiny stuff. Actually if I was doing it, I'd have pre-outs on the stereo, run a small amp and run that to the sound bar.