So I have a facelift E53 with Nav. I installed an aux connection by simply wiring up a socket to the aux input behind the head unit. It works fine, when something is plugged in the head unit allows you to go in to aux mode - however I've noticed one problem....

If I plug my iPod/iPhone in and then plug in the charger (powered by the 12v socket) then the head unit no longer detects the aux signal. I can only imagine this is due to the earth on the charger and earth on the audio link having some kind of imbalance?

Is that my problem? I noticed you have some resistors and capacitors added to your circuit. What exactly is the purpose of them and would adding similar to my aux connection resolve my issue?