I did a write up on this very issue a long time ago over at club
armada. I will see if I can find it for you, but long story short you can add it. I think for your year model, the DVD player is the brains of the operation. If you are going the OEM route you will need the DVD player, Monitor, and the wiring harness for the both, as your car does not probably have them. That said, there is another option outside of going totally aftermarket which is the one I discuss over at club
armada with the help of other members who have gone before me. It is not hard to do, but you will have to pay close attention to detail. If You can get an OEM screen with the Screen harness ( I actually have the harness for the screen for a 2008
Nissan Armada, I need to make sure that it is the same for your 2004) you can tap into it and supply it video and power. You can use anything for the video/ audio source. For my 2008
Nissan Armada I put in a Roku and that thing was awesome.
I found some images of the custom install that I did. The item is not a Roku, but a WD streaming device (They stopped making those, and that is when I replace it with a Roku). This should give you an ideal of what you can do.