03 Yukon dvd add-on

I have a 2003 Yukon with the delco tape/cd/xm head unit. It allows rear seat passengers to choose a different source from the front/main output and listen to it via headphone jacks. If my vehicle had come with a factory installed dvd system, it would be one available source

So far every audio shop, and all the gmc dealer/service depts are telling me that the only way to play the dvd audio thru the car’s speakers is with a FM Modulator. I refuse to accept their answer. Primarily because I know it can be done thru the head unit if the factory player is used. Similarly, there is an in-dash 6-CD changer that is also available. I’m not looking to add this, but in looking at the head-unit’s display, I can see symbols for the tape, cd, cd changer, traffic, xm, and dvd source. So I know that the head unit can accept those inputs – I just don’t know how to go about finding out. There is a 12- pin harness on the back of the head unit that is empty except for the phone input for the onstar system. But I don’t think it’s as simple as just running the rca outputs from the dvd player to any of these pins.

Can anyone help??? Please???