The Xcarlink website explains it all. Most OEM Head units, including the Ford 6000CD/CDC can accept, and drive, an external CD autochanger. There is a socket in the rear of the unit for plugging it in. The xcarlink appears as an autochanger to the unit and works in the same way. The radio controls allow you to select "CDs" and tracks as you wish.
The usb/sd one I had (also with line in) needed you to create a file structure with folders named CD01, CD02, CD03.....etc up to CD10. You could then drop mp3 or wma files into the folders and the unit played them in order, moving onto the next "CD" (Folder) when it was finished. Each folder could take up to 99 files.
The display on my (Citroen) radio would read CD1 track 10 or whatever. That one wasn't clever enough to give the song title. When you switched off the car it shut down with the radio and fired up from where it left off when you started up again. Depending on the level of compression you use on your files you could get enough music on a 2GB card to drive from Lands End to John o' groats AND BACK before it repeated itself! I use MP3 Pro at 64 K mostly and found the quality excellent.
2008 2.0 TDCi Zetec