I had issues with door locks and latches not working on my old Toyota Previa. The dealer quoted me hundreds of dollars to replace the mechanisms which were frozen up with corrosion and, on one side, apparently broken. On a pleasant Saturday afternoon I took off the door panel, removed the mechanisms, cleaned them, reassembled the broken one (it just had a dislocated lever), and lubed them with Muscle Grease (used by Toyota techs) before reinserting them. They worked like new and I never had a problem again.
WD-40 will dry out eventually so you need to get some real grease on the actual mechanisms or you will have a repeat of this problem. Good luck!!!