That's not an aux input...that's for the steering wheel remote control. "Auxiliary Input: Versatile-Link Ready (KCA-121B Auxiliary Input)" as per Alpine's site and owner's manual. Do not plug an aux in to that location...read your manual. This i found as well which seems like another way to add aux input though the blue tooth input...again it equires an adaptor which virtually all Alpines usually do.

 

lol my bad.. never used it just saw it there. Always just use USB. I assumed that was it because it seems like everywhere says it has aux.