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!!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car