Buick 1998 Century Owner's Manual page 82

Rear-Door Child Security Locks
Your vehicle is equipped with rear-door child security
locks that help prevent passengers from opening the rear
doors of your vehicle from the inside.
To
use one of
these locks:
1. Open one of the rear doors.
2. On the inside of the rear door will be a lock. Insert
your key into this lock and rotate it upward. This will
engage the safety lock.
To
disengage the feature,
rotate the lock downward.
3. Close the door.
4. Do the same thing to the other rear door lock.
The rear doors of your vehicle cannot be opened from
the inside when this feature is in use. If you want to
open the rear door when the security lock is on, unlock
the door from the inside and then open the door from
the outside.