How often does it do this? It points to a voltage sag. Do both window switches do it? What about headlights? Being a GTi somewhat clouds the game a little because the ballasts will provide some capacitance ( @mattarosa? Am I right?) and as such a dip in brightness might not be seen. Brake or tail lights might be a good one if someone is stood behind you.
I am still not convinced by your alternator.... or the earth. You said the battery has been swapped out so I'm assuming this is good. How sure are you of the voltages you're measuring? How did you go about taking the measurements?
I'm also not convinced you haven't got a ropey earth somewhere.