Now that is a tricky problem! The antenna for the ULF on the X5 is located above the ULF in the luggage compartment. That could be why you are experiencing better performance when the unit is in the back seat. Although the trasmission is digital, and should be either clear and free from noise or not working at all. Still; try putting the phone in the luggage compartment to see if the audio performance improves. If so, you could buy a 5M cable Fakra and move the antenna into the centre console like in the E46/E39. There is also an option of seeing if there was an updated antenna available with better performance, as the part number chnaged several times during the life of the vehicle.