Decent code reader like BMW INPA or BMW Scanner 1.4 will read the roof module and display any codes set for the system so that may pinpoint where the fault is for you. If the windscreen motor is not functioning may be broken wire in there, noticed a few with that have cropped up. Have to take the trim off to check the wiring loom inside the top of the windscreen. Good job you had the main motor relocated to the boot makes it easier to check that end of things.