I repaired my 2007 BMW Z4’s convertible top hydraulic unit by removing the entire convertible top, with the help of a great Youtube video. Others have posted that you can get to the motor by reaching it through the trunk. I tried reaching it that way and decided it was too difficult. When reinstalling the whole top, I realized that you should be able to access the pump by removing only the rear, aluminum “C-channel” that holds the rear of the top, and lifting up the rear window section. In the following video I show the procedure, although not every step (since my roof was already detached by that point).
I am pretty sure this procedure enables accessing the motor without removing the whole convertible top but I won't consider it definite until someone confirms it. Please post a comment on the Youtube page if you use this procedure successfully. Thanks!
Full notes page:
https://docs.google.com/document/d/1WVV ... sp=sharing