​​​​​​Not sure if it's the same with the R56 but in an R53 the cable needs to have a 300Kohm resistor between left and right...this resistance is what tells the head unit the cable is plugged in. So check this first if you have a multimeter handy

You may also want/need to put in place a couple of capacitors to remove any ground offset between your aux input device and your head unit.

Have a Google for BMW input cable DIY for more info

I know this as I installed a "plug and play" Bluetooth adapter into my car last week...this needed the resistor in order to work..but didn't need the capacitors as it shared the same ground as the head unit. Can't complain as it was less than $10 including postage.

 

This explains why the noise filter I put in place works as well.

 

I know this an old thread. However, I have this same issue, my AUX does not show up anymore on my display. It doesn't show for AUX VOL either when pressing the music note for adjustments. It has worked flawlessly for the 7 years I have owned the car (was already in place when purchased). I have replaced the head unit once for a CD error problem and after worked. I am an Journeyman Aircraft Electrician and until just recently worked in microelectronics so none of the wiring is difficult for me. I did recently replace the H/K amp due to a LOUD static interference issue. That fixed that... however after about a couple of weeks it happened. I disconnected the Bluetooth attached to it and reseated and it worked again but a few days ago happened again. No amount of reseating seems to be helping. After reading the above posts, I may disconnect charging to the Bluetooth to see if that helps but, I have had it in this config for a couple of years and it never was an issue. I was wondering if the AUX is strictly a function of the head unit and has nothing to do with the amp (which would be weird).
I guess it's possible that the main connector in the back of the head unit is loose or has come loose (haven't checked yet)? Is there a possibility that it can actually be the head unit itself as the issue?
I am trying to keep the oem look of the main instrument pillar section but, IF I was to get a new head unit that will work with the H/K amp; what would that be?

 

I figured out the problem. The MPOW Bluetooth Receiver has just slightly unplugged from the cord...not 'snapped' in. All good now.