Automatic A/C: Ambient sensor airflow?

[dj92]

Hi,
I'm gathering parts to give my HB an automatic A/C, sadly some things are a bit confusing.
As I have no service manuals, I have had a look at an EPC drawing which showed an air duct going from the ambient sensor to the upper section of the air box - some cm behind the flaps.
However, there's a piece of plastic not closer specified on the drawing which connects duct and air box.
As the air box is quite expensive here, I want to get away by simply connecting the sensor and the air box with a small tube, but two questions arise:

1)What is this sensor supposed to measure?
If it's the mixed air being about to enter the cabin, it seems legit.
If it's the actual cabin air temperature, wouldn't it be better to connect it to a small pc fan or the intake side of the blower?

2)Is that piece of plastic some kind of valve?