i've upgraded the stock speakers but i'm still missing something in the rear...

i'm looking to add a 2 channel amp w/2 subs to the stock system but want something other than an enclosure that takes up significant trunk space..
and i don't want to lose the spare tire either...

has anyone done this on the E70??

looking for some design ideas...

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2012 X5 50i M-Sport
2014 X3 35i M-Sport