I thought it might be helpful to add my own experience with pictures.
In UK we call it the tailgate, the back door on the Avant. It would not latch shut, however hard I banged, while the other 4 doors worked fine on central locking.
The whole trim under the window, the width of the car, from the lower plastic frame to the bottom of the door, can be prised off as one piece. It's fastened with nasty spring clips that you push back afterwards. Watch out for the wire to the small inset light that comes on as you open. If you can't unplug it (I couldn't) be prepared to prop up the large trim piece.
You have to remove this whole piece of trim to change a fog lamp bulb.
What seems to have failed is the actuator (part no. 4B9962115B) on the right (in the picture) of the lid that's connected to the lock by a long rod. In my case it had failed in the "open" position (see 3rd picture), not allowing the lock to latch when the door was closed. Compare my pic with a replacement and you'll see the actuator arm should be more extended.
I disconnected the long rod (at the bottom of the 1st pic) which then allowed the lock to work normally. I think the actuator's function is to allow separate unlocking of the door. As I don't use that function, disconnecting it was a free repair!
Pix show:
1. Door latch with rod to actuator disconnected, central locking solenoid (white) above
2. Door trim piece with spring clips
3. Actuator for unlocking boot independently, shown in 'open' position that prevents lock from latching -- rod should be extended
