Mute Wire???

[Gromet]

OK, I am going home tonight to wire up this new Phone Hands Free system for my Treo 600 Phone. the Unit gets hard wired to Constant, Accessory, Ground and there is a forth wire that needs to go to the stereo mute wire to mute the stereo when a call comes in...

Does anywone know on an 03 with the stock six disk in dash changer head unit, which wire I would tap for this mutting capability???

Thanks