Citroen Synergie Electric Sunroof?

On my Citroen Synergie, the electric sunroof won't open.
If you push the open button, it moves 2 inches (approx) then closes again. If you push the close button it does the same. If you lock the car it opens to about 2 inches and closes again 2 or 3 times. Originally the sunroof stuck open, and I had to manually wind it closed. I have replaced the motor, and there is no debris in the gulleys. It has been suggested to me it is the "motoreducer" (whatever that is). If it is that, what does it do, and how can it be replaced?