A harness buckle locking assembly for a buckle of for a car seat includes an On-Board Diagnostics (OBD) device, comprises a first transceiver and is configured to operationally engage an OBD II port of a vehicle, and a housing, which is positionable over a release button of a buckle of a car seat. A depressible button is attached to a pin and can be depressed to actuate the release button. A lock unit selectively locks the pin within the housing. An electronics unit is operationally engaged to the lock unit and communicates with the OBD device and with an electronic device of a user. The electronics unit can signal the user of occupancy of the car seat, when the vehicle has been turned off for a preset time, and also can selectively actuate the lock unit to unlock the pin.