convertible top will not retract
Are you getting a warning indicator like a red light flashing on the switch? First check the cargo divider in the trunk.
I suspect the hydraulic convertible top motor is failing or has failed. It is difficult to get to and unfortunately it is located in a place where it can get wet, especially if the small drain hole is blocked by debris. In addition, it is inside a plastic enclosure that is foam lined and the foam will soak up water like a sponge and keep the motor wet for a long time since it doesn’t evaporate very quickly inside the enclosure. The motor corrodes until it dies.
To replace the motor requires that you remove the whole convertible top with the motor and hydraulic lines. I did this job for a friend on a 2003 Z4 2.5 awhile ago and it was fairly complex but doable for a person with moderate mechanical experience and a reasonable set of tools. If you are a beginner, I don’t recommend starting with this project. Also there are times when you need two people and the location of some fasteners requires that someone get inside the trunk. It helps a lot if that person is small. The motor itself is expensive and a genuine BMW motor runs over $600 on discounted specialty websites. The dealer will run $200-300 more, I think.
Thanks, Alan. Very helpful.