2000 Ford Focus electric locks/acuator failure recurring problem

Electric door locks acuator failed first time in 3/2006. Was able to lock all doors manually. Front driver's door acuator replaced and recall for rear passenger door latch corrosion (recall work) done at same time on the left rear passenger door.
In 6/2006 acuators/elec. door locks failed again. This time the front passenger acuator was replaced also along with dr's front door again. 
In 8/2007 locks failed 3rd time. was able to manually lock 3 doors but not the drivers door. The fuse was blown, replaced and all elec. locks working.
5/2009 Electric door locks fail again. Able to manually lock all doors except for the drivers door. They replaced the drivers door acuator again. When we went to pick up the car, they checked the locks with us present. They did not work! They did more diagnostics and found the fuse blown, replaced fuse, it blew again. They found the left rear passenger door lock acuator was drawing more current than all the other doors. The aucator on the left rear door was found to be faulty. Instead of replacing, they disconnected the power to that door. After retesting, found all other doors to be operable. We will manually need to lock that left rear door. They want us to see how things go for awhile, and say that if none of the other locks fail again, they will replace the left rear acuator and all should be fine. What do think about this continual problem? Can you please advise? These acuators are not cheap to replace and since this seems chronic, I'm wondering if there could be a "mechanical" problem causing the electrical failure or if there's anything else that should be checked. Thanks for your help. Kathi