I've spoken to Kenwood about this and basically you can't get video out of the phone to your kenwood using the standard cable with STINKS! There are a few options though.
Not for the Kenwood, but generally for video is the HDMI out for the iPhone 5. Which is a good start.
For the Kenwood, a company called iSimple is apprently coming out with a composite video out adapter for the iPhone5. You can read about it here: http://www.ceoutlook.com/2012/11/01/car-audio-to-finally-get-iphone-5-video-adap ter/
I personally would prefer a solution that involves no cables. One solution I've seen is using a older jailbroken ipod touch (if you've got one laying around) connected directly to your standard DDX719 cable and running an app that basically turns your touch into an Apple TV.
There are also very small HDMI to composit video adapters that you can grab for about $25. They're powered via USB, so you can use the DDX719 USB cable to power that, connecting the composit outs to your DDX719 and your iPhone via the digital AV adapter using HDMI. The thing I like about this solution is that most of the hardware can easily be tucked away so it looks like a simple one cable plug and play when it's done. This is probably the easiest thing to do.
I'm in exactly the same situation. I got my DDX719 about 2 days before I got my iPhone 5. To be honest, I wish I kept my iPhone 4s. One other thing I'll probably try to do is get a power inverter and actually connect an Apple TV in the car. This way the wife and kids can stream video from their devices instead of having to jack in cables from all over the place. It may not work as I won't have a Wifi net in the car, but I'll play with it.
Good luck though. :-P
