Jimmy, if the car physically has rear speakers behind the plastic grilles, the chances are that the separate amplifier that sits downstream of the headunit to power them, has given up the ghost. It's a little silver Blaupunkt unit on angled mounts, located in the under-floor compartment which is under the metal cover that lies beneath the carpet in front of the driver's seat. Once you're in there it's just a couple of Torx screws and connectors to undo to get it out. Don't pay the silly prices you see for them on eBay, they're so common that someone on here is bound to have one, any of our breakers should certainly be able to help
Edit: I believe that the early cars 2000/X-2001/Y with the Chorus cassette headunit only had the front speakers, but the Concert cassette or the Symphony 6CD would come with rear speakers fitted also. From about 2001/51 when the second-generation headunits came along, all A2's came with rear speakers.