Sab,

The symptoms you described, the random closure of the trunk, and failure of the central locking system were exactly the symtoms I experienced when I had a PSE (Central locking pump) failure. The W140 has two pumps - one in the trunk (door closure assist) and one under the rear seat on the passenger side of the car operates the central locking system, gas pump lid and lumbar support on seats. The pump you will need to replace is the PSE pump under the rear seat (passpenger side of the car) The item isn't cheap. You can find a new Bosh PSE pump on Ebay for $390 - dealer wants about $750.

Check the fuses as Rosich suggests. If the pump is working you will also hear a humming noise coming from under the rear seat location when the system is activated.

PSE pump failure is common in our model of MB.

Russell