The instrument cluster display indicates which rear seat belts are being used.
[a343] - rear seat not occupied or seat belt not buckled
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The warning lamp lights up briefly when the ignition is switched on. The warning lamp lights up again briefly if the corresponding rear seat belt is buckled/unbuckled. If the seat belt is unbuckled while the vehicle is moving, you will also hear a warning tone.
Note
For further information on the seat belts Link.