Mystery harness behind dashboard

My model of MX-5 is: 2010 NC.

Installed a new head unit today, all works well, but whilst doing so I discovered an unconnected white harness behind the dashboard. Any clue as to what it is please? I left it be but just curious why it’s not connected. As far as I am aware everything works in the car.

Did your car have the 6 cd changer?

Either the 6 cd changer as already mentioned OR the aux in near the 12v cigarette lighter.