Zune's USB doesn't output audio, nor does it let you browse the file system. Or at least, it won't without the Zune software on PC, which means car stereos won't detect it.

Your best bet is maybe some kind of headphone jack to Bluetooth adapter like these

https://www.amazon.com/TaoTronics-Bluetooth-Transmitter-Receiver-Simultaneously/dp/B01EHSX28M/

https://www.amazon.com/TaoTronics-Bluetooth-Transmitter-Connected-Transmission/dp/B074QLB1Y7/