Toyota Yaris Owner's Manual page 143

WARNING

After returning the rear seatback to the upright position
Observe the following precautions. Failure to do so may result in death or
serious injury.

Make sure that the seatback is securely
locked in position by lightly pushing it
back and forth.
If the seatback is not securely locked,
the red marking will be visible on the
seatback lock release knob. Make sure
that the red marking is not visible.

Check that the seat belts are not twisted or caught in the seatback.
NOTICE

Stowing the seat belts
The seat belts and the buckles must be stowed before you fold down the
rear seatbacks.
3-3. Adjusting the seats
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