craig, the AUX appears when the left/right channels sense 300 kOhm resistance between them. it means your cable does not have that resistance... or, the specified resistor is placed between wrong pins... that part number - where did you come up with it? the AUX is wired differently on different models, and even same models with diff option... yeah, yeah, i know, there are only three wires, how many versions can you have - believe me, the BMW explored all of them... lol...

how good are you with electronics? wires? your harness should have 1 brown wire, ground... that one run uninterrupted... the other two wires get "shorted" to each other with a 300 kOhm resistor. and then, 2 22nF capacitors are placed in line on the 3.5mm jack side...

is yours wired like that?