If you are connecting the shuffle to the USB port in a car audio system, it is using the shuffle as a storage device with song files on it (like you connected a USB flash drive with song files). Whether it works or not depends on whether the car audio system can access a shuffle and find its song files. You'll need to consult the documentation for the car audio system to get a list of compatible devices.
OR, you can buy an inexpensive USB flash drive, fill it with song files, and use that with your car (just leave it connected instead of constantly connecting and disconnecting your iPod). Here's one for about $10 with four times the storage of a shuffle, that barely sticks out of the USB port
http://eshop.macsales.com/item/SanDisk/SDCZ33008G/