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Pretty sure that no C5 was ever factory equipped with an Mp3 connection. So it must have been installed by a previous owner.
But as to your question USB plugs usually carry 5 volts of DC power as well as L&R audio signals but an 1/8" Stereo plug will only carry the L&R audio signals. So while you may find an adaptor it will likely carry only the L&R audio signals but no power for the USB device connected to it.