Car stereo usb connection.?

I`ve got a new car stereo with a usb connection and i thought that i could just plug my mp3 player straight into it, to listen to my tunes. but the mp3 just goes into file transfer mode and won`t play. the stereo plays from a memory stick fine. any answers, cos its really bugging me!!its a cheap one that i bought off ebay, stupid thing to do!! the instructions are really poor and give no guide to the usb.
Answer
It seems your mp3 player is not compatible with your unit, check on the web for compatibility.