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.
Ensure that the rear seat backrest is engaged. To do so, pull forcefully on the seat backrest.
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 for the second row of seats are located on the rear side of the two outer rear seats on the second row of seats. For the third row of seats, use the Top Tether anchorage on the rear side of the seat backrest on the third row of seats.
upwards More. 
under the head restraint between the two head restraint bars. 
into Top Tether anchorage 
without twisting. . 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 .