If the rain sensor is removed for windscreen replacement, it cannot be re-used. This is stated in the Alfa ELearn manual. It probably wont work if refitted with gel, etc. Sensor uses IR light reflected from the front surface of the glass, through the sensor/glass junction, and this has to be optically neutral. Replacement part no. is 46548671, about £100. The alfa GT has all the cables installed for the rain sensor (and the misting sensor) just under the courtesy light cover. Though you can retrofit a sensor to this, Alfa seen fit to not include the electronics in the wiper/lights/indicator switch console. This electronic module can't be bought seperately, you must purchase the entire switch stalk unit where it resides, of which there 12 variants. Part no 156040578 is the one with everthing, VDC (steering wheel rotation sensor), Steering wheel radio control, cruise control and Winter pack, with the electronics for the rain sensor, plus an output to front headlamp washers if fitted. Other versions of this switch unit have variations on the above four options, but they would all cost the same as a spare part, which could be higher than £200. See FIAT ePER
This and the difficulty of removing the Steering wheel, makes retrofitting the rain sensor expensive and tricky. I picked up a rain sensor hoping it was a straight plug and play, now I will have to create my own receiver for the sensor signals and tap them onto the wiper motor cables, the latter being the easy bit. I've yet to test the sensor output to see what signals it sends, its a 3 wire affair, 12v, earth and an output line, which will either be an analogue signal proportional to rainfall (easy to detect) or a serial pulse stream, which will be a challenge. I might just sell the original new sensor and fit one of these kits instead.
Rain Tracker Rain Sensing Windshield Wiper Controls