The wiper rain sensor on my 2001 Passat has never worked properly. It seems to only randomly respond to rain. My VW dealer has worked on it several times, but usually they do not even identify what is wrong since it will work for a few minutes from time to time. Most recently, they replaced everything but the main mirror mount where it is located. I guess I must be really patient, since I have put up with this for so many years, but it appears that they will never be able to fix this. I contacted VW but they will not respond to technical questions. I have to work with my dealer, whose service work is generally excellent. Two questions: Does anyone else have problems with this sensor and does anyone know of a simple way to bypass it so that I can at least have intermittent wipers (there is not setting for this option with this model)? Thanks.
I feel your pain. I have had a 2004 volvo S80 and a 2008 S60. Both cars had rain sensors that irritated the hell out of me. Adjusting those things is like trying to win an argument with my wife 
Try locating and covering the sensor with tape. Maybe it will default to normal intermittent operation