Im not much help here, bacause I have the BA system, but on mine, without nav, and single CD, the deck has a digital output to the amp, and the amp does the signal processing duties, then spits it out to the speakers via analog signal. So you couldnt splice into the digital signal cable, youd have to do it after the amp. Problem is I dont know if the basic system is similar to the BA...I think they share head units though so its likely?
I can tell you where the BA amp is, and you could easily go see if you have one there too. Go to the rear hatch, and lift up the spare tire cover. Towards the front right of it, by the rear seat, under the carpet is the amp. If memory serves you can get to it just by lifting the carpet out of the way, or maybe it was plastic, either way thats where it is. Go see if you have one there, and what variety of wires go into and out of it.