Just a thought, but Scala bluetooth stereo single user units are down to 100 bucks. Bluetooth IPod stereo, with really good sound quality, no distortion in your Zeppelin at 100 MPH... pretty nice.
Plus you can switch tracks with a button on the side of your helmet. Really.
No install, no hassles... hmmmm.
Now if you want to have a stereo install on your bike for fun, its not a substitute, but I've used helmet based sytems for years and the sound quality is amazing AND it was a lot easier than trying to install a stereo.