The 54-pin adapter plug is part number 84.12-0 308 375. I would build your custom wiring based on this.

I think that the caller voice output is line-level, because that would normally route through either the radio or AMP, depending on year and options of the car. I don't think that ever drives speakers directly.

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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