If both of those options don't work, the potentiometer is the most likely source of the problem. I used a multimeter to test the faulty switch in the Audi, and I briefly shorted the corresponding contacts: both motors moved. Therefore: I ordered a new potentiometer from eBay and that should fix it. (The one I received was from an S2 - I noticed that right away ).
Downloaded the wiring diagrams. My English skills aren't bad, but I still couldn't find the section on headlight beam adjustment. Maybe someone can help me with that?