Lift the motor-gearbox-unit a little bit upwards so that you can insert the black element with the nose into the hole of the white element. (marked in green)

Insert the nose of the black element into the hole of the rotary switch, making sure it is oriented as shown.

Gently move the rotary switch so that it fits into the slot in the housing. (marked in blue)

Then you can tighten the upper housing part with the five screws (TR15).