Rellik: a 1cm-square piece of cloth with a pair of wires clipped to it will work as a rain sensor for the circuit I posted. And yes, your top will work with the key off.
Lea, I might have used a micoprocessor or microcontroller, but I wanted something that anyone could make themselves within an hour or two for very low cost.
A trim potentiometer could be used to tweak how long the output holds for the windows. But I really don't think it is necessary. The hold could be made long enough to accommodate even the slowest of windows, and if it is on longer than necessary for faster windows, nothing is lost.
Regarding the glitch you described, at some point in the mid-90s the top drive was modified slightly to improve the chance of latching. I can't remember the change, though. It is probably very easy to have the automatic operation ignore the fact that the top is not completely latched at the windshield and continue on anyway.
I'm attaching the most recent revision to the design. The diagram shows the most complex circuit: the one for the 1-button remote. For the later 4-button remote, the 4 gates to the left of the diagram are not needed. I wrote-up some descriptions of circuit operation here:
http://www.benzworld.org/forums/showthread.php?p=1897471&posted=1#post1897471.
I expect to have the parts to build a prototype in a few days. I still need to work-out the resistor and capacitor values. If you or anyone else has some suggestions for improvements, I would much welcome them.