I'm sure this has been discussed in this forum as well as in other forums, so I hope to kinda give a summary of what I have been able to gather on this topic. Maybe it'll spark something in someone to try something. Well, anyways, here goes:
The orginal way to achieve nav/tv was to use an RGB converter (purchased from Audio Authority). You would basically take your PS2 or Xbox and use component video cables (purchased from Monster Cable) and connect it to the RGB converter. The RGB converter would then be connected to a switch, which had to be spliced into the video cable coming out of the oem navigational unit located in the trunk. The switch, when turned on, would allow signal from the PS2 or Xbox to interrupt the navigation signal, and thus show video. Nothing would happen to the navigational unit and so it would still run properly, even while you were watching video.
The next way simplified this process. PAC introduced a unit that basically combined the converter and the switch together and uses a standard video out instead of a component video cables. Basically, you take your PS2 or Xbox, connect it to the PAC unit and then splice the PAC unit into the same video cable as mentioned above. The unit is operated with a switch and the same results are achieved.
The last way that I have found comes from a company called VAIS Technology. They are in the process of developing a true plug n play system, which will allow you to plug in a unit directly into the back of your stock navigation head unit, using all oem Toyota connectors. The company mainly focuses on Lexus/Toyota, but more on Lexus right now as most the owners own Lexus'. I have been talking with some of their engineers and have been trying to work out a test fit for a 4Runner. It's still in the early discussion stages, so please don't get all excited and start including this in your letter to Santa. If you want to find out more about it, check them out at
www.vaistech.com.
The biggest difference I think between the first 2 ways and the third is that the Vais unit is plug n play (i.e. no wire cutting period) and it completely integrates audio output as well as video output (the other 2 units only incorporate video - audio is usually achieved by fm modulator or some other way - there is actually another PAC unit that does this seperately).
Anyways, that's pretty much nav/tv in a nutshell. There are a couple of other companies out there saying that they can do it, but it all pretty much boils down to a rehash of the PAC unit.
So yeah, just my 2 cents.