Coastal Technologies has a device called Ultimate Lockpick 3 that provides a NTSC composite video and stereo audio input for the 2006-2007 Prius. I have used one on my 2007 Prius to display the video output of my Icom IC-7000 Amateur Radio transceiver, and to play the audio through the car speakers. Your AUX input will continue to work, and you can use it for a second radio or your iPod.
The 2007 Prius still has an NTSC composite video input. This input is part of the Prius, not the Coastal device, it's a separate connector on the back of the multi-function display from where the Coastal device plugs in. The Prius does not require GVIF as reported elsewhere, and I'm not sure it uses GVIF at all. To feed video to the multi-function display, you must have some device on the AVC-Lan signal the Prius display to read video from the input. That's what the Coastal device does.
The Coastal device masquerades as a DVD video player. This allows it to use the multi-function display's three DVD video-width settings. "Wide 1" works best with the IC-7000.
The multi-function display contains firmware to access a number of different devices, regardless of whether your car comes with those devices. Two known devices are a DVD player, and the navigation system.
The AVC-Lan technology is used on multiple brands and models. The Lockpick 3 will work with some Lexus SUVs, but not the ones with a rear-seat DVD or CD/DVD changer. I guess it uses the same AVC-Lan ID as one of those devices.
Cardemand (an eBay merchant) has a similar device, for about $100 more, which sounds as if it includes a frame-store and thus may play video that the Prius display would not otherwise lock to. At this writing, they don't seem to have an online presence other than the sale pages in their eBay store.
For more information, see the AVC-Lan paper