Originally Posted by Moments

My solution was to buy the cable parts from Maplin and just make the Aux cable. I can't recall exactly the pieces but there was a resistor across the two channels, something like 300kOhm, which allows the head unit to know that Aux is an option. I think both channels were de-coupled with something like a 0.22uF ceramic capacitor, typical audio stuff, and the ground was common. Cost around £6.

If the L and R channels don't have a resistive connection, "Aux" will not show in the menu on the head unit.

The key discovery for me with Aux is that on install the volume level is low for safety reasons. With Aux selected, use the buttons 1 through 6 to get the right level.