I got myself one of these:
http://www.electric-avenues.com/amplirider.htmlIt is a three input, one output audio amplifier. I plug my MP3 player, Zumo 550, and sometimes my iPhone (only to hear it ring) or sometimes a police scanner (when traveling). You can set each of the item's volume's independently on each item. The Amplirider output is connected to the AUX audio input on the I/S. It is powered; I have a fuse block mounted in the fairing, so power connection is easy.
I mounted mine using double sided 3M body moulding tape to the upper triple tree clamp. I pull the rubber flap over top of it for weather protection (not that it gets that wet up there anyway) and it's hidden out of the way, but I can still get to the volume knob on it easily, even while riding.
Works great and is a quality item.
*EDIT: Sorry, I re-read the original post and saw that you wanted to do this through the radio. Left the post up anyway in case anyone wants to do this through the AUX input.