A neutral safety switch in a Ford vehicle prevents the driver from starting the vehicle when it is not in park or neutral and activates its backup lights when the vehicle is in reverse. In Mustangs with C4 and C6 transmissions, the switch is on the side of the transmission.
In Ford Mustang FMX models, the switch is at the shifter. Fords that do not have backup lights do not have the backup light plug and provision. Vehicles with automatic transmissions have a neutral safety switch as a safety measure that stops the flow of electricity from the starter when the transmission is in gear. When a neutral safety switch is not working properly, the vehicle does not start, even though the battery is charged and there is no other mechanical problem.