It's not really your phone, but more a limitation of your car's stereo/ head unit. Your stereo is looking for UMS device, i.e. Flash Drive to read music from where your phone isn't a UMS device, but a USB MTP device. It won't be able to read the music from the partitions. I've tried with my Galaxy Nexus (also MTP) and all it does it charge the phone when I enable USB Debugging.