If your car stereo accepts a USB flash drive, but never seems to play your MP3s in the proper order, this post may be of interest to you.
When you plug a flash drive into your computer, the files on it only appear to be in order because the computer reads and sorts them before displaying them on the screen. When files are written to the drive, they are not necessarily written in order or even from the beginning of the drive. This is done to prolong the life of the drive. As a result, if your car stereo isn’t programmed to sort, it will play the MP3s in the order in which they’re found. (This is also the case with some MP3 players, though fancier ones like modern iPods probably sort.)
I use an application called DriveSort to correct this little annoyance. It will sort the files on a FAT-formatted drive (most flash drives) so that they can be read in order.
Be aware that this application is to be used at your own risk. It is recommended to back up any data on a drive before running DriveSort on it, especially if that data is sensitive or important. I, nor the author of the application, take any responsibility for the loss of data or any other sort of damage. I haven’t had any problems with DriveSort, but better safe than sorry, right?