When the master vacuum switch on your door lock system begins to leak it can prevent the lock side, the unlock side, or even both sides from properly functioning. You can replace it with a new part but it is fairly expensive for such a simple device.
To that end, this guide will help you to attempt a repair before springing for the replacement part.
Guide credit goes to Nick K., and helpful member over on BenzWorld. He graciously agreed to provide the content for this guide.