Then you need an aftermarket head unit or a switch that you can kit on and off as you wish. Otherwise, like most all other cars the Mazda 3 only has accessory leads, there are no wires that will have 12 volts on them only when the head unit is on. I will admit this is still an assumption but one I am very confident in.krstofer said:The lighter wire is one of the accessory wires. I only want my amp to be on when the stereo is on.