It's in the left rear compartment. Lots of stuff there, but the amp is mounted vertically right against the outside of the X.

Do you have DSP? If you do, you won't like the amp being bad...expensive. Aftermarket isn't simple either, unless you like running wires from front to back, etc.

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual