LKN624

Hey guys, just wondering if someone can help me or knows...I have the Original Subaru Kenwood GX401EF2 Head Unit in my 2006 STI. I want to add an Aux connection to it (like the 2007 STI in the US came with a Aux Port in the Ashtray with a Pigtail to connect to the back of the Clarion Unit). There is a circular female socket on the back of the Kenwood GX401EF2...and I have no idea what its for. See below. Its the black connection port on the bottom right of the head unit. Is it an Aux or CD-Change Port or what is it exactly?
Thanks in advance!

Don Clark

https://www.scoobynet.com/scoobynet-...ax-iphone.html

CD Changer in port as shown on page 14 of attached manual