The BMW compatibility says that it supports “Non DSP only”. How do I know if my car has DSP or not?
Grom has a page on DSP as they’ll have you covered
DSP stands for Digital Signal Processing and is used to enhance sound quality. Dension products will only support cars that do not have DSP but as mentioned Grom do. Most BMW vehicles released in Australia do not have DSP, with the majority that do being released between 1997-2001. DSP equipped vehicles normally have a video monitor where it will show DSP as an icon in the display.
In a DSP vehicle, the nav unit is connected to the radio (analog), the radio to the DSP amp (digital), and the CD to either the DSP amp (digital) or the radio (analog). Note that the screen in the front of the car is simply a dumb terminal that passes commands to the 4 units in the back: radio receiver, navigation, cd changer, and amplifier. The output from the radio unit is 2 channel (front) only and does not vary with volume. All modifications to the sound (volume, fader, treble, bass, DSP equalizer, etc) are controller by the amplifier which then provides 10-12 channels of amplification (depending on M-Audio or not).