It works like most other gas powered Mercedes with a PSE pump. Mercedes are kinda different in this department, so it doesn't work like most other "cars" with you'd normally find an electrical solenoid to work the power locks. It works on vacuum and pressure applied to vacuum servos in the doors. The PSE is the pump (which is located under the back seat) which provides the vacuum and pressure.
Some older MB's use engine vacuum which is stored in a vacuum canister.