350insp on Benzworld took these pictures of the cassette player in the MCS: Note that the circuit board on the back of the cassette player exactly matches what's in the C-Class so we can probably use the same hack to get an aux-in port onto the ML. Here's the C-Class mod pictures from lovin_my_C230K on MBWorld (remember to put a cassette tape in first so that the system thinks a tape is playing):
Adding Aux-In to MY2000 M-Class