You'll need to be more specific...
USB is as the acronym suggests a bus (universal serial bus). It is used to connect devices together. I presume you mean want to connect a USB storage device containing music to a head unit?
If so what sort of car do you want to put this in? What does it have currently? What other features do you want/not want? How much are you will to pay?
Most even low end head units have a USB interface port that you can plug in any old USB storage device to play music from. Alternatively there is likely a solution out there that will add a USB interface to your existing head unit.
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