The swith is located just under the rubber between the rain sensor and the roof. The wires keep the swithc in place, but you can use glue. As I said before we can connect to power of the remote to the 12 V of the car, This mod is available with the DIY - Clown nose blinking LED. But if you are leasing the car, it's better to change to battery once a year!
To open the rain sensor, just use a credit car to separate the two parts. It's very easy also to close.