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I have a 1999 528i with a factory installed rear winow power sun shade. It is stuck in the up position. When I press the switch, I can hear some faint clicking noise from the motor mechanism but no movement. It was in this mode about a week ago and then miraculously started working, but now is stuck in the up position again.

So the quesion I have to the experts ... is there an easy way to fault isolate the failure without pulling out the back seat (btw- I have fold down rear seats) to then remove the rear deck lid to eventually get at the mechanism and motor assembly?

I also read somewhere that a bmw owner indicated that the system can get in a mode whereby it is in a reverse polarity situation? If so, how can this be resolved? I tried removing the fuse, disconnecting the battery in hopes of resetting the system but to no avail.
 

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I fixed one a long time ago.
If you can hear the motor running there is a gear inside the drive reduction box that slips on the shaft. It shouldn't be slipping.
IIRC there is a current limiting circuit that knows when the shade is at the end of it's travel and shuts power off.
You need to take the motor out and see what's happening inside.
Not sure about the reverse polarity you mentioned, however you could switch the motor leads to get it to go down.
 

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