* Brake lights: When the brake pedal is depressed.
* Running lights (or parking lights): When the headlights are turned on (usually automatically also with the parking lights) or the parking lights are activated separately.
* Turn signals: When the turn signal stalk is activated, the taillight on that side will flash. The opposite side may also have a running light that is less bright than the running lights to indicate a turn.
* Hazard lights: When the hazard lights are activated, both taillights will flash simultaneously.
* Reverse lights: When the vehicle is in reverse. While typically separate, some configurations may incorporate reverse light functionality into the taillight assembly.
* Faulty wiring: A short circuit or other wiring problem could cause the taillights to malfunction, staying on even when they shouldn't. This could include a blown fuse.
So, in short, they come on as a safety feature to indicate the truck's presence and intentions to other drivers.
