Seems like the trouble with water soaked soft top motors is quite common. July this year, my soft top would not open. When I pressed the opening button, it unlatched from the top frame of the windscreen, but would not continue the movement into fully open position. When I pushed the close button, it latched into fully closed position. My local dealer fixed the problem by replacing the soft top motor, and the repair did not cost more than $ 2000,-...... :!: When I saw the replaced electrical motor, I realized that the failure had nothing to do with my use of the soft top. The motor was in a very bad condition with a lot of corrosion. After having done some research and read several posts on different BMW Z4 user forums, I made a written complaint to BMW. Have not received any answer yet, but I am afraid it will be negative as the car is not under warranty any longer.
During my "research", I found what I think is the original service bulletin (SI B 54 03 03) for this problem http://tech.bentleypublishers.com/servlet/JiveServlet/download/56-29644-288775-4559/540303g.pdf After have read all the posts regarding this obvious design error, it strikes me how reluctant BMW is to acknowledge the fact that they have a failure in their design. BMW show a worrying lack of attitude towards the customers purchasing their prestige products.