Klaus - If you pull the trunk side out and pull the hose, you will see the problem - the fitting at the bottom of the drain hose is just about the same size as the hose - they pinch the hose to push it in the fitting.

The drain will constantly clog if anything of any good size goes down it!

I checked the trunk drains every couple of weeks with a cup of water. I used a piece of high temp auto wire - the stiffness was just right. I was able to put a small bend on the end of the wire so that it would make that first corner. I kept the wire right in the trunk. I marked the length to the bottom fitting on it.

I considered enlarging the hole in the bottom fitting, but my wire worked so well that I was lazy.

Keep the trunk seal and where it lands clean - mine used to get debris in it - possibly from wind on the highway? - as this will keep the water out, especially if you park on a level place.