number6,

I reinstalled the original headunit, hooked a voltmeter between the serial line (open because cd-changer was removed) & ground. The line voltage held around 13v. When I pushed the onstar button there was a transient drop of approx. 1.5 to 2v then the sound came on and it went back to 13v. When on pushed the onstar again (to turn it off) there was a transient drop of 1.5 to 2 v. Is this enough voltage change to use the mute line on the riocar and not worry about the bitstream? In other words, can the mute line be programmed to detect changes other than 0 to 12v and 12 to 0v?