How can I fix the central locking system on a 1999 Holden Vectra CD?

I have a 1999 Holden Vectra CD. The rear left door central locking system no longer works on that door, but works with the manual lift/press button. The cable harness from the bottom of the door to the pillar looks to be in good order. I have also tried different settings with the red child safety lock next to the lock mechanism, but still doesn't have any effect using the remote locking/unlocking button. The window up/down button on the door works alright. Can anyone help please?