There are many AMI cables sold on Ebay and Amazon, some are made not to work due to mis-configured the iPod detect resistance. For iPod and USB drive to work, the iPod detection resistance must be 18.7k ohms (pin 21 and pin 22), for the AMI to be in the AUX External Audio Mode, the resistance must be 1k ohms or less than a minimum detection specs. If your MMI 3G doesn’t come with Bluetooth audio streaming as default, you can make the cable into AUX external audio mode and connect a bluetooth audio dongle to stream music from your phone.
Now a day, there are many CarPlay/Android Auto systems out there that required AUX external input for the sound to work. Some vendors don’t include an AMI AUX cable which we may have to modify our original cable to make it AUX external audio mode.
All we need to do is to replace the original 18.7k ohm resistor between pin 21 and pin 22 with a 1k ohm. If you plan to use the iPod or USB drive in the future, get a 1.2k resistor, run 2 wires out of the AMI and connect across the original 18.7k, this will bring the total parallel resistance to 1.1k. When you don’t want to use the AUX any more, just remove the 1.2k resistor and the original 18.7k will be back.