I am in the middle of doing mine.
Remove the wipers, first marking their rest positions on the screen with bits of masking tape.
Then you need to remove the black panel that borders the bottom edge of the windscreen. Remove the two plastic fasteners (one at each end), then, with a good scraper, remove the black panel (called a scuttle cover panel) by separating it from the windscreen - you are breaking the seal between it and the w/screen. You'll need to be fairly energetic about this, but take care not to damage the panel! Mine took about 25 minutes to free. Then the panel withdraws backwards - there are four or so pegs through the fwd bulkhead that just disengage.
Before going further, mark the position of the drive arm where it attaches to the motor by pencil onto the plate beneath.
You should now see the 6 bolts, all needing a 10mm socket, that secure the wiper drives and the motor. Remove these and withdraw the whole assembly as one piece.
Disconnect the electrical connector, which is behind the motor.
The motor then separates from the drives by the one central (13mm) and three more 10mm nuts. Again, note the position of the drive arm relative to the motor shaft.
You should find the cable clipped by a cable tie.
That's as far as I've got. I haven't yet got a replacement motor, as mine may be fixable.
When you replace the motor, connect it electrically and allow it to run to rest. Then connect your wiper mechanism to the motor in the same position it was in when you took it off. This should give you the right range of movement for your wipers.
Attach the whole back onto the car.
Next reattach the scuttle cover panel with a new seal. I believe Citroen sell these, but I shall probably try using a black sealant.
Finally reattach your wipers.
Navy Tiff, November 2008