I just got my 2003 Toyota Sequoia back yesterday from having a Z2 installed. It has the factory DVD system. We had anticipated a tricky install from reading OEMValue.com (our original choice was the Toyota system) but in looking around for the best value on the Z2, various stereo shops told us different things, the general consensus being that it was no problem. Two shops told us that they would convert the factory plugs to RCA.
The shop that we eventually bought from initially thought that it would not be a problem but found out otherwise. They got it to work by taking apart the factory unit and using the "brains" to integrate with the Z2. On brief testing, all the functions seem to work. The wireless headphones work, you can also listen to the audio over the speakers. The installer said that you could control the audio from the backseat unit, as well. I haven't tested it myself. I plan to spend some time going through it function by function today.
I am a little disappointed with the steering wheel control integration, though. I plan to get the little black Pioneer steering wheel remote for the Bluetooth phone functions so maybe I'll just end up using that more if it is more functional.
All in all, it beat buying a brand new Sequoia just for the Navigation. I love my Sequoia, even more now!