Stillen did an ML320 with headrest monitors in them (check back issues of Car Stereo Review). Thinking about doing mine...MBs monitor option works on 2000 and later models (mine is a 99).

I don't know what model MB you have, but some headrests aren't large enough for the monitors...you moght consider the seatback area? I guess a couple of grand is disposable for you, but I'd have to think about it, especially if it was just for show only!!

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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