Hi Rainer,
Today, I finally got to visit my parents, and I also brought my digital camera to document it for you.
Unfortunately, it turned out that my memory had deceived me; that stupid thing doesn't have a blind plug at all! What I had mistaken for a blind plug in those old and bad pictures was actually one of the mounting points.
I also took a new series of photos with a modern camera (at that time I only had a blurry webcam), which you are welcome to use if you ever want to document the effects of damaged LED streetlights on the site - the damage is very clearly visible.
But unfortunately, it also means that there is no blind plug available.
I'm sorry that I can't help you with your question.
Here's another recent picture (resized, the original is MUCH larger) of the part:
Is the dashboard layout in the A3 the same, or does it look fundamentally different from the start?
Best regards,
Jan.