Most of the systems in the truck are tied to a "class 2" serial bus which end in the ALDL connector (for OBD-2 diagnostic hook-up). The onstar mute command is on a different serial circuit which seems to contain all radio controls (including proprietary controls to control aux. cd-changer). Therefore it makes sense that the Onstar mute command uses the same tyoe of serial circuit as the head-unit to cd-changer commands.