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My car has factory I Pod & USB connectors to play music.
But when I connect Droid X to the USB port, my car stereo will not play any music. Anyone know how to fix it.

its really confusing, im using a clarion stereo and i was able to get music to play but no shuffle mode and make sure everything in one folder, now if i only can get slacker to play via usb that would be real great
To get anything else to play besides MP3/WMA or whatever format your radio will decode on your SDcard, like Slacker, Pandora, you will have to NOT use the USB and use an analog signal minijack-minijack, most new radios have a 3.5mm input they call "AUX" - but you would have to control it from your phone verses if you listen to music off your SDcard you control it through the car radio, folders, subfolders, ect. - I prefer it that way, just plug and play, put the phone away, and it charges at the same time too.
Just to clarify, the Droid X does not output audio via USB. Setting the Droid X to use USB mass storage mode allows your car's audio system to read the music files stored on the micro SD card and play them, just as if you plugged in a USB thumb drive containing music. Like labeledbass said, if you want the Droid X to output music, you'll have to do it through the 3.5mm audio jack (or perhaps microHDMI, haven't tried that yet).
If a car has USB input it likely has bluetooth audio streaming too.