Remove the four (cross-head) screws securing the motor mechanism to the housing. Use long-nosed pliers to ease the cover off the housing, moving from clip to clip. There are five clips. Unclip the small plastic access cover on the bottom...
Get a secondhand genuine one rather than a new aftermarket one. Some of the aftermarket ones seem to be for Left Hand Drive, and can't adjust far enough to be useful.
(Of course there's no guarantee that a secondhand one will not have been replaced with an aftermarket one at some point in its life - guess how i worked that out!)