Does the monitor have IR capability? Little LEDs that you can see through a clear or smoked lens? If so, the audio would go to the monitor, and the monitor would broadcast IR audio. And yes, for that you would need wireless headphones.
The other thing to figure out is whether or not it's somehow wired into the factory stereo, through an aux input or something. Could it be, or can you trace the RCA audio wires to the monitor?
As for a mobile DVD player that can go under a seat, there's a ton out there, just google "mobile DVD player." I'm installing a Myron & Davis setup in my Nissan Armada in the next couple of weeks, so I just ordered the M&D AD218.