DAVIDA64,
I have a Honda Civic with Nav system which has a PCMCIA slot in which I an adapter and a 4gb SD card in which i play MP3's from. Sounds similar to your setup minus the Hard Disk.
It will not play iTunes music store M4A files. You must first burn the songs to a CD using iTunes. After that set your iTunes import settings to "MP3" and whatever bit rate you want (I use 160k.)
If you re-import the CD right after you burn it iTunes will remember all the track information (song titles etc.)
After that I usually just delete my DRM ridden iTunes originals in favor of the non DRM MP3s.