I have hooked up a Z1 GPS/DVD/Radio in my 04x and have hooked up the factory DVD so it will play sound through radio to car speakers. Or it can be played with headphones for the kids without going to speakers. If I want to play a dvd on dash radio I need to plug in the Video cable into the exterior video input on the Factory DVD - this allows dvd or map to display on the factory Dvd player.
It took about six months to get the info to figure out the factory DVD, finaly got the info from a great DVD factory expert.
The rep sent a PDF attachment with the info for hookup. Basically you have to make a loop from DVD back into DVD. It normally works by sending loop to factory radio and then radio back to DVD. Without factory radio you need to make it a loop at the DVD, and run a set of speaker wires (via RCA plug) to your new radio input with the sound will play through your new radio to your X speakers.
Since it is an attachement send me your email and I'll forward it too you. I do also have some pictures of my install I could send as well. It is all his work and method, not mine - it works.
I wish there was a way to get video from new radio to factory Dvd without external input, but I could not figure that one out. Took the DVD unit apart and just don't see working - not just a wire fix.
Send me your email for the pdf instructions via pm