I have a schematic for the cell phone, but it is dated May of '92 and may not be the same. But I imagine it is fairly close if not identical to '96.

The pair of 12-pin connectors are shown. One goes to the telephone handset cradle, and the other goes through a connector to the Transmit Receive Unit.

The schematic shows a "Cellular Telephone Radio Mute Relay" module. If you were going to try to use the ex'g wiring to patch-in an additional audio input, I think this module would be required. Is it included with cars that have no factory phone? I don't know. But at any rate, it handles just one audio signal, so you surely wouldn't want to use it for stereo...