There could be a couple of reasons why its not working for your car stereo.
Look up the manual for your stereo. Several have a size limit as to what they will accept.
Example: It will read up to an 8 GB drive.
Also make sure you have the right music file type it will read.
Usually, Stereos will read FAT32 and not NTFS. Make sure you have it formated correctly.
Worst case scenario I have seen is that some model stereos will not read every brand USB drive.
The manufacturer may have a list of approved drives.
Hope it helps.