how can i fix the central locking on my ford focus?

Can anyone help I have a Ford Focus 1999 1.8 Zetec Ghia.
When I bought it the back near side door lock didn@t work off the central locking yet the other doors did. Now though however none of them work off the remote unless I lock the back doors from inside and to lock the drivers door i must first lock the front passenger one, also the only way to lock it with the key is to do sofrom the passenger door otherwise the drivers side just springs back open. I dont believe it to be the fuse as you can still hear the motors working. Any ideas or advice would be greatly appreciated.