In the E38 and E39, the ibus is readily available on the connector visible behind the CD changer access panel (E39) or LH rear taillight access panel (E38).

I-bus = white/grey/yellow (the only wire with 3 colours)
Ground = brown
+12V = red with green stripe

I normally just openthe panel, and connect to the wires using crocodile clips straight into the back of the plug. No unplugging or dismantling necessary.

In the E46 it is less accessible - you may need to remove the trunk trim side panel to gain access to the wiring...?? (I haven't worked on many E46s)