mom-of-3

I just purchased a 2008 Tahoe LTZ with dvd players in the headrest. It also has the nav screen. I was hoping there was a way I could play one dvd on all the dvd players with the sound going thru my radio. We are assuming the dvd players are aftermarket (correct?). I can put a dvd in the nav area and it with play there but not on the rear ones. I have to put a dvd in the rear player but can only get the sound to work thru headphones.
I have no owner manuals for this and the very small dealer I bought it from does not know either. Can i play the rear dvd players thru the radio. Can anyone help me?
Thanks in advance.