Ford Focus Wagon Tailgate Lock Cannot Open By Remote

I'm new to here, hopefully I'm posting at the right place and thanks for any help in advance.

My 2005 ford focus wagon tailgate cannot open suddenly, I cannot open the lock with the remote, cannot open it with the button on the dashboard, and cannot open it with the button on the tailgate, the only way I can open it is inserting my key to the key hole on the tailgate. The remote works for all other 4 doors, and I can hear a tita-tita sound under the dashboard when I click the tailgate button on the remote.

After google for long time, I think problem is my tailgate lock motor. I spent another two hours to remove my plastic trim on my tailgate, took a close look at my tailgate lock motor, cannot see any problem, unplug the cable and then plug it again, problem is still there.

bought a multimeter from Canadiantire today, check the voltage between any two pins of all that 4 pins in the connector(connect to the tailgate lock), and clicking the tailgate button on the remote at the same time, I can see voltage between on pair of pins get changed from 0.9v to 1.1v and then go back. other pair of pins looks like no voltage at all(xxx mv). Is this correct? I thought the voltage should be at least 12V when I click the tailgate button on the remote. (I can see 14v voltage between the Pins for tailgate wiper)

If the voltage is not correct, what can I do for next step? I did my best to checked the wires(as long as I can see) but did not find any problem.

the di-da sound under the dash looks good, every time I push the tailgate button on the remote, or tailgate button on the tailgate, I can hear the "di-da" sound for one time.

Any other potential problem? how about fuse? I can see the fuse box, but don't know which fuse is for the tailgate lock, and don't know how to check if it's good or bad.

Help!!! Thanks for any ideas.