You talking about car audio? Anyway, first thing to note, MP3 is
just a digital format of a song used by computer to store or play
them. Typical MP3 players now can play WMA format too. Yet another
digital form of a song/music that Microsoft came up with to compete
against MP3.
In terms of car audio, most play CDs/burned CDs (RW or W). Now they
even comes with harddisk memory that can let you encode your favour
CDs into the headunit so you can play directly without the CD. Of
course quality is compromised since you are actually compressing
the songs, but general public can't tell the difference or doesn't
mind.