The PCI bus is essentially a superset of J1850 VPW protocol. You should be able to monitor the bus and see what info you can get from it.
If you have remote volume up down control's on the steering wheel hit those while monitoring the bus and you can identify the command that get's sent. This should help track down the "ID" for the radio module and then you can just monitor all messages for that particular ID.
Paul
www.obdpros.com