If you see a lock solenoid, that is indeed strange. If it is a US spec car it was delivered with central locking actuated by vacuum and pressure; vacuum to lock, pressure to unlock. There is a hard plastic line going to the lock actuator in each door, not a rubber hose.

Check for a central locking pump in the spare tire well. It will be wrapped in a thick layer of foam. Have someone turn the key back and forth in the driver side lock and listen/feel for the pump to activate for a couple of seconds with each turn of the key. If it doesn't, the problem could be a signal to the pump. If the pump activates, see if the downstream end of the line connected to the pump holds a vacuum. If it doesn't, you have a leak in the system. If it holds a vacuum, there's a problem with the internal switching of the pump.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD