05 chevy HD 2500 w/ OEM overhead

I need to know how to get the audio signal from the panasonics factory installed head unit.  i was thinking at the very worst splicing some RCA's into the the audio wires and running the signal through an fm mod.  has anyone had any experience with this or know any color.

P.S. i'm obviously using an aftermarket head unit AVN2454 to be exact but the customer doesn't have the aux input plugs.

follow up.  i didn't find any audio wires in the only harness that lead toward the front.  the other harness's out of the dvd player are, 1 for input a/v and on for the moniter.  it seems as though the audio signal my be digital going to the head unit.  but just a guess 

the moniter does however have IR repeater that work with aftermarket ir headphones without any further modification.  so i'm assuming that if i spent more time with it i could have found low level audio from that harness.