If the rear seat backrests are not locked, they could fold forwards in the event of an accident, heavy braking or sudden changes of direction.
As a result, child restraint systems cannot perform their intended protective function. Rear seat backrests that are not locked can also cause additional injuries, e.g. in the event of an accident.
If the seat backrest is not engaged and locked in place, this will be shown on the multifunction display on the instrument cluster (fold rear seat back electrically) or the red lock verification indicator will be visible (fold rear seat back manually).
If the child restraint system is equipped with a Top Tether belt:
The risk of injury can be reduced by a Top Tether. The Top Tether belt facilitates an additional connection between the child restraint system attached with ISOFIX or i‑Size and the vehicle.
The Top Tether anchorages on the second row of seats are located on the rear side of the backrest of the two outer rear seats. For the third row of seats, use the Top-Tether anchorage on the rear side of the seat backrest.
upwards More. 
under the head restraint between the two head restraint bars. 
is installed, route the Top Tether belt downwards between the load compartment cover and the seat backrest 
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of the Top Tether belt into the Top Tether anchorage 
without twisting. . Always comply with the child restraint system manufacturer's installation instructions when doing so. downwards More. Make sure that you do not interfere with the correct routing of the Top Tether belt .