I would'nt go screwing around with the NAV system either.
An alternative set-up is AMG's S55 (NY auto show) which features video screens behind each headrest, and a DVD system integrated with the factory audio. It was absolutely fantastic. The system is built entirely by Alpine, very well done. Go to their website & check it out.
Apparently this is a dealer installed option available on any Mercedes.
In looked into the same Alpine setup for my LX470, and they can do it. I don't know if the M5 headrests are big enough, but they have all sorts of different tv setups. Impressive, but overkill IMO. If one were interested, I'd go this way. On another note, a friend of mine (former apple ceo Mike Spindler's son) has a Chevy Tahoe inwhich he has 5 dvd players and separate screens, etc. etc. If you REALLY want to do an impressive job, I can get the contact information for the people who did his.
C'mon guys ease up! Half of this website is full on antecdotal information. I have not confirmed my sources unsolicited comment, but think there must be some truth to the "rumour" since several of us have heard something similar. I will ask my friend if he can find out more about this.