As may not suprise people, I did this to my ML500
I wanted it fitted as if it was from the factory, and was told it was impossible ..
It was a big job ..
front seats out
rear seat bases out
centre console out
carpet out
right hand lower boot trims out
(Now you have access to all the relevant wiring ducts)
disassemble top of both seats and unhook leather from top to install brackets for monitors, and cut holes in leather for monitor supports
install cabling in seats for monitors (two cables)
rebuild seat and replace headrests with new ones (different shape)
install monitors on seats
install main wiring loom
re-install carpet
install RSE controller (involves cutting hole in carpet)
re-assemble everything else
code using Star Diagnosis
I also installed reversing camera and TV tuner at the same time, additional work: -
left boot trim out (upper & lower)
right hand upper boot trim out
drop headlining at rear by 10cm to access wiring
install wiring loom for TV tuner, reversing camera and TV-Tuner->RSE controller
rear door panel off
replace left hand wiring loom in rear door for one with TV & Camera wiring
install 3 TV antenna amps (tailgate window & two side rear windows)
install camera in rear door handle
put door back together (MUST use new door panel clips, 14 of them)
code using Star diagnosis
About 2 days work, with the seat trim being done by a professional seat person. BUT , the big thing was the cost of the parts. The two monitors, the TV tuner and the RSE controller are serious money. And the antenna amps were well north of £100 each.
So, if you don't care whether its actually factory original parts i'd go with an after market option. Possibly even the Mercedes one with a centre screen.
Cheers
Richard