However ...

if the CD player in your car is able to play aac files (you say you simply copied them and that works) it's not totally unlikely at all it will be able to play these mp3 files also (although they might be copy protected). Worth a try to simply copy them to a CR-RW and see what happens.

If it doesn't work, do what Ed says: burn as audio CD. The disadvantage: an audio CD contains 70 minutes of music at most. A cd with aac or mp3 (depending on the quality) can contain 700 minutes, so you might prefer that.

Kees