[xonxoff]

Depending on which chime interface you had installed (Soundgate, Pac) there is a gain control on it for the chime and one for stereo/onstar.

I am still trying to find an interface for the steering wheel control that will allow me to control the volume of the Onstar from the wheel control

Have your installer tell you where they hud it. Mine is hehind my glove box so I can adjust it.