'05 Escalade - Mute factory radio/NAV?

Is there an easy way to mute the factory radio on an '05 Escalade?  I'd like to wire a cell phone kit to mute the radio the same way OnStar does, if possible.  I don't want to kill the power to the radio, because that would shut down the nav system everytime the phone is in use.  I also don't want to use relays to interrupt all of the speakers, because that seems like a lot of work, but I guess that's what I'll have to do unless someone has a better idea.

Thanks for any help.

-Michael