I'm not sure anyone has answered your question directly yet. I believe the stereo HU and the climate controls share a common circuit board, so aftermarket replacement of the factory HU will be a complex exercise, if not impossible. No one has done it yet, to my knowledge. Some have installed aftermarket HUs without removing the factory unit. Most of the info posted is for the Bose system. I have the base stereo, and have not confirmed that it, too, is convoluted with the climate controls. I suspect it is.
jaak is referring to use of the SAT inputs to connect to a MP3 player. Unfortunately, the existing interface requires a digital 'handshake' with the SAT before the analog inputs are enabled. Check his web site for lots of pics of the hardware under discussion.
There is almost certainly a place somewhere in the factory HU where one could wire in a manual switch and switch in an external source. I have done this on several factory (GM mostly) units in the past. Since my wife is primary driver and is satisfied with burning CDs for playing in the MO, I have not been motivated to do this mod yet.