How can i fix my Focus Mk 1 tailgate lock problem, please?

The central locking on my 02 1.6 Focus Zetec will now only operate the passenger doors but not the bootlock. The dashboard release switch doesn't open it either. I have to use the key. I guess there's only one fuse for the central locking so that's not the problem because the rest works. Can somebody please advise me how to use my multimeter to test whether the fault is with the continuity of the wires to the lock or with the solenoid that operates the lock? Any other ideas would be really welcome. Thanks