I have the older style multifunction steering wheel on my 2004 MCS.

I know what all of the buttons do with the exception of the middle button on the back left side. All of the back buttons control the stereo in some way. However, the middle back button on the left side does nothing. I can push it in any mode -- i.e. CD, Radio, or Disc Changer -- and nothing ever seems to happen.

What is this button used for, and is mine broken?

 

If you have the sunroof and ordered with the passenger side ejection seat that left middle button is the release.

Actually, I have the same button and don't know. I forget if the left or right middle button switches between radio and CD.

 

I think the button that switches between the radio and CD is the middle button on the right side...but I'm still curious about that pesky left button!

 

The Owner's Manual does not list any function assigned to the middle left button.

 

Yeah, the left middle is a vestigal button to ensure symmetry with the right side. No known use, though Mikey at newministuff.com can tap into the Kbus? and utilize it for various purposes.
Me, I think it should be a mute button, which my middle right button served until I install the ipod into the Aux Input.