I have a GTA now!
Join Date: Oct 2011
The sensor that hangs behind the screen, about an inch from it, is a mist sensor.. so that the auto demist will work.
The rain sensor, as far as I know, is part of the windscreen... its the little eye you can see from OUTSIDE the car, above the mirror.
I was told, when i replaced my windscreen, that the sensor comes with the new screen, its bonded onto it. And in fact if you look closely, you can see that the sensor is part of the windscreen - and accounts for the unbelievable cost of rain sensor windscreen replacements compared to standard screens. In Australia a standard screen is about $150, the rain sensor version $700 - $800.
I find that replacing wiper blades sorts any rain sensor problems.. also i moved to the Bosch Aero's and now the system is flawless.