When you say you put them on DVD, the car will probably only read a CD. Very different beast, even though they look similar.
When I do mine i use a rewritable disk (wipe when I am bored of that lot...) and just copy the files to the disk, I don't create an "mp3" disk. That may not make much sense so here goes...
I use Roxio Easy media creator for burning disks. I have the option of creating a "mp3" disk which puts the mp3's on the disk along with a playlist file. My car CD palyer doesn't work with these. What I have to do is just copy the folders of mp3's to a disk like they were any other files i.e. I create a data disk.
Hope this both helps and makes sense.
Best wishes,
Woppa