After further research:

The complex communications between Onstar GPS, cell phone, OBD-2 etc. uses a Class B (Type 2) serial circuit (http://www.intel.com/design/auto/autolxbk.htm). The only other serial circuit in the truck is the one I am interested in. It contains a mute signal for onstar to the headunit and cd-changer as well as the proprietary controls from the headunit to the cd-changer. That appears to be it. This circuit can be interfaced independently from the type 2 circuit at the ALDL and is probably "more primitive" than the type 2. It is probably a 160 or 8192 baud data stream as described at http://www.techedge.com.au/vehicle/default.htm Is there a simple way to figure out which one it is?