It looks like you need to buy the Infiniti/Nissan switcher, and then splice it into the OEM system. I don't have the stock Navi system, so this is the part I'm not sure of, but you need to connect a DVD player to the Navigation Video Switcher, and I'm guessing the OEM one won't work. If you're looking for MP3 playback, you'll need something aftermarket anyway. So I think you're looking at the cost of the Navigation Video Switcher plus the cost of a new DVD/CD/MP3 headunit plus the cost of installation.