If your vehicle is showing taillights or nighttime running lights are completely out, here are the most likely causes and the parts you'll need to fix each one.
A blown fuse is the most frequent cause of all running lights going out simultaneously. This is designed to protect the circuit and is often triggered by a momentary short, a failing bulb, or trailer wiring issues.
It is surprisingly common for one taillight bulb to burn out unnoticed, followed weeks or months later by the second one, making it appear as though the entire system failed at once.
Poorly spliced aftermarket trailer hitches or lighting accessories frequently chafe against the vehicle frame, causing a direct short to ground that instantly kills the taillight circuit.
The main lighting switch on the dashboard or steering column can wear out internally. When this happens, it may successfully turn on the main headlights but fail to send power to the separate parking/taillight circuit.
The relay responsible for switching power to the parking and taillights can stick open or suffer internal coil failure, preventing power from reaching the back of the vehicle.
Taillights rely on a solid ground connection to the vehicle's chassis. Rust, corrosion, or recent bodywork can sever this connection, disabling the lights even if power is reaching them.
In modern vehicles, lighting circuits are controlled by a central computer (BCM). A failure or software glitch in this module can disable the taillights, usually accompanied by other strange electrical issues.