Actually, with some of the newer head units with USB ports they will play MP3's off a straight USB stick, and the Ford Sync it can read like a USB stick.
Try plugging it in and mount like a disk drive. It should read music on the SD or the internal. I set up a USB Stick for my Dad on a Ford with the Sync, because an iPod would be way too complicated. Works OK and it was simple.
If it does not work let us know. I'm curious.