I do not know about sunroof drains, but I had a lot of trouble with the trunk drain of the '90 744TI that we had, the drains clogged up all the time.
In the case of the 740 drain, the fitting that the hose fit into was too small, causing the hose to have a crimp in it.
After you get things cleared up, I have two suggestions.
First, on a regular basis, pour some water into each drain to see if it comes out the bottom.
Secondly, I took a piece of high temperature wire (the insulation was stiffer then regular wire) and cut it to the proper length, to use as a clearing tool for the hose..
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