citroen c3 pluriel roof problem?

Hello,
i have a citroen c3 pluriel 1.4 hdi (year 2004)

I am looking for the position of the sensors , specially the sensor
who feel the closure of the roof.
I have this problem:

i washed my car, then waited for about 1 hour to let it dry

then i opened the roof (about half), and cleaned the car internally

Then, when i closed the roof, it reached the closure position, but the
roof engine didn't stop!

I heard a sound like "trrrrrrrrrrrrrrrrr" coming from the rear window,
so i stopped to press the button

Worried, i tried again to press the button, and again i heard
'trrrrrrrrrr' from the rear window....

I suppose there is a problem in the sensor that realizes the closure
of the roof...

but the question is: where is it?

Thanks!