Probably you have problem with the encoder.
Are you converting your own Cd's to MP3?
What year is your car?
I understand that only from 2005 on can be able to play MP3 when they retrofit the car i do not remember the build.
Mine is a 2005 and after the dealer reprogramed they play MP3 Cd's from the CD player or DVD player without problems, but on that year they are picky about the encoding, also I use Verbatim or TDK.
They can not Play MP3 from the CD changer
Do not make a multi session, create a folder in your computer put inside the subfolders of several mp3 converted already, sometimes i put 10 full cd's encoded to 191kbs or i use 320 but probably fits about 5 cd's on one disk, then burn that as DATA and close the Disk.