It not only sounds promising but I don't think there are many codes being sent over this circuit. The only equipment used by this circuit are the 1)headunit 2)aux. cassette/cd-changer 3)onstar mute Since there is no main CPU directly controlling this circuit if I remove the cd-changer (which probably has the most complex codes going back & forth to the hu) I should only have left initialization codes between onstar and the hu and codes when onstar is turned on & off. That sould be it.....or am I missing something.

Interestingly, on a side note, the speaker signals from onstar splice into the aux. player speaker wires and run to the hu. I'll bet the muting signal from onstar is the same signal that aux. cd/tape players use to activate the hu to accept there speaker signals. I just mention that because there are people in other threads who would like to play there riocar thru there stock GM hu. Since this would require the riocar to send out a signal to the hu I assume this is more complex than our plans on this thread. Just thought I'd throw it out there. If someone thinks it is possible, I'd being willing to play with it while trying to solve my initial goal. Thanks so far everyone, this has been a real learning experience!