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On the early 129s there is a switch in the bottom of the door that signals to the soft top control module that the windows are all the way down, very simple. Until it has this signal from both windows, nothing else happens, as the glass in the full up position will get broken by the top operations, is my understanding. If window work is done and the small paddles which touch these sensors are out of adjustment you can end up with a non-functioning top. Usually pretty easy to correct, once the door panels are off.
Later ones, they went to Hall effect sensors in the window mechanism, which tells the control module how far along the windows are. It will at least allow the header latches to release. These sensors are also used for the window drop-down when opening/closing the doors as you have noticed. Hassle with them is if the battery is disconnected the sensors have to be resynchronized.
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