there are several posts regarding adding an aftermarket HU in the map pocket. you wouldn't input the sound through the 16 pin connector, but rather splice a harness into the factory radio harness. you would also disconnect the speaker wires from the factory HU.
you could always use a stereo input aux-in like the AuxMod (tape button) or AudioLink (cd button) and wire it up through the aftermarket front channel outputs. i haven't read anyone trying that. you could always give it a go and post a how-to.
there are several posts about aux-in devices including stickys in these electronics section.