See this post for the pinouts of the radio. (The radio is the component with the CD. The MID is not part of the audio path.) http://www.xoutpost.com/articles/x5/...formation.html

If you disconnect the DSP amp, the radio will automatically revert to normal four-channel stereo output. The radio plug will not have the rear channel pins installed, but you can easily add them. The pins with a short wire pigtail are part number 61.13-000 7 449. Here's a post that illustrates me adding missing wires to one of my older cars.
Alternator Whine Fixed

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input