Handsfree cell through factory stereo/speakers

I have a 2002 Suburban with a factory sound system and On-Star.  If I use the On-Star as a mobile phone, it cuts off whatever I'm listening to (CD, radio or tape) and outputs the sound from the phone through the truck's speakers.  I'm currently installing a Nokia handsfree kit (CARK-91) and I'd like to replicate that feature so I don't have to clutter up my car with that extra speaker.

The car kit has a "radio mute" line out and the speaker out is just a 1/8" mono headphone-style jack.  I have no idea where the radio mute line needs to go or where I can splice in the audio output from the car kit.  I have been unable to locate any wiring diagrams for either the stereo or the On-Star unit, and I'd appreciate any info you guys could provide.

Thank you.