Sorry, I don't write German...

I had the same problem, which was fixed by my dealer within 2 minutes!

The sealingrubber of the rear wiper sprayer was not correctly fited.

With heavy rain, water got inside the car at the point the water spayer tube leaves the car to go to the wiperblade.

Rain water poured inside this hole, along the tube for the spayer, down into the boot.

As you describe, only on the right (where the tube of the spayer is).

Via the cover of the right back lights, water got inside the boot of the car.

Actually I could have fixed it myself, if I would have noticed what the problem was!