Hi eyoung,
I have a 2004 GX470 with Mark Levinson/Nav but rear audio only, not DVD. I recently had an Invision G10 DVD/TV tuner installed. It is their latest 10 inch drop down DVD, not even on their website yet.
http://www.invisionindustries.com/products.html
The audio plays through the car via a hard wired FM modulator that integrates with the factory antenae wiring (much better sound than the wireless FM modulator on most systems). I cannot control the video from the Nav screen but gnetcanada.com in canada sells an external video integration interface box that should allow me to do watch whatever is playing on the rear screen on the nav screen. Here is the link.
http://www.gnetcanada.com/navvideo-models-..._rgb_toylex.asp
The picture quality is excellent, even off to the side. The sound qaulity is also very nice. (It also comes with two wireless headphones so you don't have to listen to the DVD.) I haven't really watched much TV yet but I hear that it is not usually that good Another nice feature is a USB port that will let me play home DVDs or picture slide shows from a portable USB drive. I can use the remote from the front seat to turn off the unit.
One downside is the obstructed view through the rearview mirror when the screen is down. I just use my side mirrors while our little one watches his Sesame Street DVDs.
One problem I have yet to sort out is loss of the ability to use my CD changer since the install. I think this has to do with an interupted circuit when the rear seat audio display in the headliner is removed to make space for the DVD player. I am looking into a workaround for this.
I got a good price (half off list) from dealerfit.com but i see they no longer have this unit for sale. I was thinking of getting the 7" headrest monitors but I really like the larger 10" widescreen size of the drop down unit and the better sound quality of the hard wired FM modulator.
Just some thoughts from my experience.
Tim