I think the amp/radio box(s) is in the trunk and the MID in the dash that looks like the radio is actually only a display.
There is probably some really good info and wiring diagrams in the TIS or Bentley manuals.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)