Yup! I've done the retrofit, joining my microphone to the MULF. I followed
this guide when it came to the wiring. Go to step 3 and look at the instructions regarding connector X14087. It's exactly the same in the 1er. When I was doing the install in my car, it appeared that the pins coming from the microphone connected to nothing, so I followed the guide to move them in order to connect up to the trunk. Additionally, like the guide, I switched pins in the trunk from the TCU connector to the MULF, since I was adding the $644 option.