The problem is that your car supports Audio CDs and you've created a data DVD which is totally different. You need to use a CD+R and use the Audio CD option found in your burner. Based on the number of Audio CDs that I have created along the way, you can add ~18 MP3 files on an Audio CD.
Also, when you're burning a CD, make sure to choose lower speed than normal to avoid playback issues and increase compatibility and reliability.
