I own an au series 2 xr-6,and to-day i got a low siren battery warning when i unlocked the car.I managed to pull the siren apart,only to find that it is impossible to replace just the battery.does anyone know if there is a way this can be done?Or is the only other alternative is to buy a complete siren from ford?

The batteries can be changed, just need a heap of those batteries, a soldering iron and some heatshrink. If you are contemplating a new one from ford you may as well buy a whole new system. If you are uncomfortable changing them you may like to find an electronics repair store around town to do it for you.