Mercedes has always used vacuum operated door locks. A long time ago, they were mechanically operated. Your system is an electrically operated vacuum pump. Your pump may be bad, or one of the electrical switches may be bad.
I have a W124, and the pump is located under the rear seat.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".