Dual audio inputs with auto-muting

[mui]

I have an 04 MCS with Harmon Kardon equipped with the Aux input. I just installed an XM Roady2 and am looking to install the Tom Tom Navigator using my Dell Axim X5 (PDA).

What I would like to do is to hook both the XM and the PDA up to the Aux in (or some other way) on the stereo with something in the mix that would mute the XM when the PDA has something to say. Does anyone know of a product that can do this or is there some other way with the HK head unit to accomplish this?