aux function on the 2005 zx4 ses 6 cd mp3 player

What is the aux button for on the deluxe 6 cd mp3 player system in the 2005 Focus ZX4 SES? I can't find any audio input lines on the stereo,
but it doesn't make sense to me that a manufacturer would put a button on the radio that serves no purpose. Hopefully someone more familiar with the Focus audio options can resolve this issue. I searched this group for any discussions on this, and only found one thread from last year that appears to leave the issue unresolved.
Dan Pilloff
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