To clarify, the rear lock is the only one that is not working.  Also,
we can hear the mechanism trying to work (clicking noise), but doesn't
work.

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The lock may just need re-aligning rather than replacing.

This will not be the problem but the answer is yes. Unfortunately it is
the most difficult fuse to find. It's on the _back_ of the fuse panel
located inside the car - and it's carefully hidden in the wiring on the
back of the panel :(  On a European focus the fuse panel is behind the
glove box and on a north American model I believe that the panel behind
a cover somewhere in the drivers foot-well.

The fuse is required to open it the lock, not close it.

I have the same problem with my 2001 5 door. Sometimes the lock
physically sticks in the open position. The lock mechanism is a two part
affair. The first bit rotates in when it hits the catch and the second
bar then move across to lock the first part. On mine this second bar
gets stuck back. Sometimes if I keep pressing the tailgate release there
will be a click and it will free its self. Sometimes its get the
screwdrivers out. It probably happens every six months or so and it only
takes me 10 seconds to free it.

To free mine I rotate the main catch in and then give a gentle prod to
the locking bar. I really should go and buy a new lock.

Graham