There is actually a very small vacuum blower that pulls air through a pipe across the sensor. In my 260, the sensor is in the panel with the Sunroof, and the fan is behind the glove box. Test it with a bit of tissue over the sensor end of the air pipe.
the suction is very slight. Suction will pull the tissue against the pipe. turn car off and tissue should fall away. No suction, check for a broken pipe connection or power to the blower motor or bad blower.