Hi there, if you are sure the alarm sounder backup battery is at fault π it is located in the rear of the vehicle on the driver's side (pictured of the unit below) the battery is inside the unit. Also, be aware if the car has lost battery charge recently the battery in the alarm sounder will need to charge for about 15-20 minutes, this can be annoying as it will still go off. So I would suggest taking the car for a drive rather than annoy your neighbours, hope that helps?