I mounted my box under the driver seat. It is easy to access and you can hear the speaker fine, even with music.
Also, you can use an external speaker if you wanted to hide the box but still have a speaker outside.

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2011 ///X5 M (On Order)

2009 X5 4.8 M Sport Space Gray/Black Fully Loaded

2007 GMC Yukon XL Denali Loaded and Customised