That USB connection is likely not using the correct profile. I believe on the dot it's simply for power and charging and you're trying to use it for USB media storage which the Dot doesn't do. The USB audio source on your car is meant for devices that store the audio files on a usb mounted storage. Most phones have a special profile for USB mounting that facilitates this but the Dot wouldn't. You might be able to do connect the dot to your car's Bluetooth audio.