My some-day plan for that part is to cut off that whole back portion to make room for a subwoofer that extends to the back seat and act as a shelf/cup holder for the back passengers. Seeing that drawing just kind of re-ignited the dream!

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