Aftermarket HU & rear dvd player

Hi everyone, I just bought a 2004 tahoe it has bose,onstar,xm,and the rear dvd player. I want to replace the headunit and i know i need the onstar adapter dont care about the xm but how do i get the stock rear dvd player to go through the aftermarket headunit?? Any ideas?

I'd call Crutchfield. They know this stuff. I'm sure there is an adapter or something similar. They have adapters for steering wheel controls.