2008 Subaru Legacy Aux installation

[Juda]

Hi! I recently bought a 2008 Legacy that seems to have every optional extra except for the Aux input, the one thing I really care about. Having done a bit of research I decided to go the OEM route and I ordered the part that I believe is the correct one. The part number is H6210AG000. I have installed it, but my head unit doesn't seem to recognise it. I have followed the instructions for the Kenwood head unit. All the cables were in the expected location and everything fits. My head unit did differ a bit from the one in the manual since mine has navigation but it didn't seem to make a difference for the cables. 

Now, with everything installed, nothing happens when I press the Aux button. I have made sure to plug something in and I even disconnected the power from the head unit for a while to reset it to factory settings but it made no difference. Any ideas?