Now i have this central locking problem on my focus 1999 all the time, after alot of looking around i discovered a few quick fixes. First the way i read to check your door mechanism is to lock it from inside and lock the doors using your key (remember to do both sides with key i tend to find when mine goes faulty my passenger front and rear door wont lock from the drivers door or but will do from passengers side) if the door actually locks then the mechanical mechanism is fine and youve got an electrical fault probably one of the wires inside earthing out or been rewired wrong. you can obviously just swap the multi_block connector for your other side and see if it locks then (ive been told that the connector wear out not too sure about it thou) it may blow your fuse again but at least you know its the wiring inside your door.

But for the other member whos multi_connecter is connected but door not locking theres a quick fix if nothing is damaged. Just open any door and press the door sensor 5 or 6 times really fast until your indicators flash,. Then go round, open and close each door your indicators should flash once for each door including the boot and buttons on your key.
If any dont then the sensor on that door is broken and this is what is stopping that door locking.
After youve tried every door leave them shut for about 1 min and your central locking unit will reset itself (your indicators will stop flashing when you open your doors) your rear door should lock now. Ive have to do this everytime it rains coz my passenger doors go funny. water must be getting into one of the door electrics some were but cant find it.
HOPE this helps mate