Is your player actually capable of playing MP3 Cds? Does the spec say it will?
You will find it easier to use the Nero Express interface. There you select the burner & media type, then Music & MP3 disc. That way there's no confusion as to what you're doing.
If your car player can play MP3 & you create the CD as I've suggested & it still doesn't work then it's probably down to the CD media. I always use Verbatim Pastel or Super AZO as these will play fine on my car stereo that's not supposed to play my own audio compilations.