how do I fix my merc A160 central locking fault?

I have a 1999 A160 avantegarde and the central locking does not work. Fob or key will not lock all doors. When drivers door is unlocked by key and opened the other door lock buttons go up to unlock but they have to be pushed down manually to re lock. Also the internal lock button on the dash does not work, neither does the auto lock feature.
I suspect the central locking pump or motor are they one unit, where do they live on the car and how do I replace them or repair them.
Richard

richard siddell
March 2010