My dad had a similar problem with his 1995 E320 driver's door. The dealer replaced the door lock mechanism, but the source of the problem was a loose electrical connection under the front passenger carpet. Unsnap the carpet/sound insulation insert and remove it from the car so that you can inspect the electrical connections to the hidden black box that bolts to the floor. The oxygen sensor cable runs from the driveshaft tunnel toward the passenger door ... ignore that one.

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Fred Hoelzle