Mercury Mountaineer 2003 Owner's Manual page 142

Each time you use the safety seat, check that the seat is properly
attached to the lower anchors and tether anchor. Try to move the child
seat from side to side. Also try to tug the seat forward. Check to see if
the anchors hold the seat in place.
If the safety seat is not anchored properly, the risk of a child
being injured in a crash greatly increases.
Seating and Safety Restraints
141