I bow to your correctness.WillemBal: Chris, I'm not sure that this part belongs to the Phaeton. The valve plug in the case of the Phaeton is the small one and has a tiny oval hole. The big round one is part N101388 and has no valve functionality at all. Did your dealer install this one in your car?
Folks, please ignore my main-dealer-supplied list above, they just went round the parts bins and pulled out stuff that looked as if it fitted.
The big round one in my list is a Golf heater duct relief vent and the oval one is listed in everything from Skoda to Bentley as a tie-down hook cover - but not listed for Phaeton.
I wonder what the difference is, it's the same part I think. Maybe it's because the Phaeton isn't supposed to be tied down at that point, so they provide a different part description... Who knows...