If it detected your connector without that 300kOhm resistor, then it sounds to me like you lack those DC blocking capacitors. So...if you haven't already done so, you should add some series DC blocking capacitors on each line on your self-made kit. I understand that these capacitors are on the aux kit that you buy from BMW. The blocking capacitors will prevent any DC signal from your portable MP3 player from over driving the inputs of the head unit and vice versa. The values of those capacitors WILL determine your frequency response...especially the lower cut-off frequencies.