These locks are intended to help prevent children from accidentally opening the rear doors. Use them on both rear doors whenever a child rides in the rear seat of the vehicle.
If you slide the child safety lock to the lock position before closing that door, the door cannot be opened from the inside. The door can only be opened by pulling the outside handle.

Locking, Unlocking with Door- Lock Knob
Trunk LidMazda 6 Owners Manual: Rear door garnish installation
Note
Double-sided adhesive tape has already been attached to the new
rear door garnish.
Warning
Using a razor with bare hands can cause injury. Always wear gloves
when using a razor.
1. When installing a new rear door garnish, follow the procedure below:
(1) Remove the adhesi ...
Mazda 6 Owners Manual: Automatic transaxle removal/installation
1. Disconnect the negative battery cable.
2. Remove the battery and battery tray.
3. Remove the aircleaner component. (See INTAKE-AIR SYSTEM
REMOVAL/INSTALLATION.)
4. Remove the front tires and splash shield.
5. Remove the under cover.
6. Remove the steering gear and power steering pipe. (See ...