How does one convert an external hard drive from ntfs to fat32 without creating a partition?

I have a pioneer fh-p8000bt car stereo with an usb connection, and supports ext. hard drives up to 250G, but has to be FAT32. Another factor is that the stereo will not support a drive that has a partition on it. Is there a way to convert from ntfs to FAT without creating a partition using Vista?