Testing will need to be performed to see what is going on with the system that operates and controls the door locks. It could be individual lock motors in question, or the system itself. Without proper hands on testing, it's all guesswork.
RepairPal Certified shops can diagnose and repair your 2002 Mercedes-Benz S500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.