Rear bike racks are ilegal to a point ... ( unless it has integrated lights ) when you have a bike on you cant see the lights and the reg .
Go thule for roof mounted
There is a chance they are incorrectly codded
There is the info how to
http://wiki.ross-tech.com/wiki/index.php/Audi_A3/S3_%26_A3_Cabriolet_(8P/FM)_Tweaks
In each corrner of the sunroof. You need to open and have a look for 4 holes. It drains to the bottom of the doors , front under the hingess and 2 more on the back
Sunroof drains
Rear wiper washer pipe runs there that causes a leak
Pellenium chamber drains blocked
Roofrails washers failed but you would see some water on the headliner