A simple cost effective solution could be to get a powered 10" or 12" sub such as a Rockford and just T splice into the low level inputs on the rear deck speakers. I did this on my E39, battery is right there for power. Of course it is not as aesthetically pleasing as a custom built box, but for $200 ish, you are done and it's pretty powerful. Just a thought.
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2018 M550ix|Carbon Black|Dinan Modifications|2003 540i|Jet Black|M-Sport|2021 X5 Mineral White
Prior BMW's: 1987 325e|1993 740i|1998 740il|2001 530i|2002 745li|2007 750li|2006 750li|2001 530i|2012 535xi|
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