Do you have the OEM subs in the rear? I think mine sounds fine.

As far as rear sound distribution, that effect is nulled out when the top is down.

It is difficult to get great sound out of the rear of ANY convertible without some real enhancements...I think MB did pretty well...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car