Audio Systems: aux input, interface plugs, ford radio

Question
2005 F350 Ford Truck.  My stereo has a aux button but nothing hooked to it.  The only thing I can find in my manual says that it is used to toggle between FES/DVD and AUX modes but nothing about how to connect something to it/them.  What does the FES stand for and how easily could I connect a jack for a mp3 player or ipod.  I asked about it where I got my remote start system installed and was told I would be better off purchasing a new after market stereo system.

Answer
you can add the Ford auxiliary input made by PIE FRDO4-AUX,
this interface plugs into the back of the radio and allow you to input audio using the rear dvd input, Ford radio needs to have a AUX button and be satellite ready
follow this link to the part you need but call before ordering, to check
ford aux adapter