// StartVehicleHorn
void START_VEHICLE_HORN(Vehicle vehicle, int duration, Hash mode, BOOL forever);
Sounds the horn for the specified vehicle.
vehicle: The vehicle to activate the horn for.
mode: The hash of "NORMAL" or "HELDDOWN". Can be 0.
duration: The duration to sound the horn, in milliseconds.
Note: If a player is in the vehicle, it will only sound briefly.