is your car's CD player compatible with the type of media you used? (CD-R CD-RW etc. . . ) Some, especially older or cd decks that come with you car just dont recognize recordable CDs.
Also, in iTunes preferences, make sure that you don't have Data or MP3 selected under disc format. If you do have, it would have just made a disc of data or MP3's that your CD player can't understand.