Anyone know what this 3rd gen head unit female connector is?

I have a 2016 with premium tech. package. It's a 6spd manual so therefore without the JBL system. Below is a pic with a "missing" plug.

I recently replaced the stereo minus the head. I used a DSP. When reinstalling, I could only find 8 plugs. Wondering if I'm only supposed to have 8 instead of 9 and/or if anyone knows what the 9th is?

Thanks

 

I'm sure someone knows for sure, but I'll take a stab at it.
Could it be for the sub you don't have?

 

It's not an unreasonable stab at it. I had a 2008 with the JBL, but I never cracked into the head unit. I believe the JBL's run everything off of an amp located back by the factory sub. Not sure what all that separate amp receives from the head.