TO #26
No weld is so tight that water cannot penetrate. For this reason, all hollow sections in a car have holes built in to drain water. If they become clogged due to dirt or overzealous application of anti-corrosion protection to the underbody, the penetrating water can no longer drain and, as a result, corrosion of the car from the inside begins. Particularly dangerous places are the body side members.
Clean the holes regularly with a tube cleaning brush ("brush").
If the vehicle is equipped with a sliding sunroof, the drainage holes in the roof should be additionally checked, see above.
For cleaning - from the front at the top, from the back at the bottom - the old speedometer drive shaft is suitable.
(Information obtained from this resource: audimanual.ru)