I just checked out that plug. It's located behind the seat, starboard side. This is a photo.
View attachment 558696
I'm showing it here as an upside down "T". The vertical part of the "T" is the negative side. The horizontal "top" (which is on the bottom in this photo!), is the positive side fused through the ACC 15Amp fuse and powers on and off with the ignition key. (Yes mine has the other half of the fitting/cover which I've removed for clarity in this photo.)
I've looked briefly, but haven't found the connector for sale. In fact, I've stopped looking. I've come to the conclusion that I'm not impressed with the weather resistance of this fitting and, when the time comes, I'll be replacing it with Weather Pack type fittings as described below.
Understanding GM’s Most Common Plug Options