Let's not forget that it is a sensor and as such, water droplets must be in front of the sensor in order for it to send a command to wipe.

There is never a perfect distribution of rain drops on the windshield so there could be times when there's more water in your view than on the sensor itself.

If only there was a rear window rain sensor...

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11 X5d JB/Cinnamon w/Bamboo/Premium/Tech/Cold/Rear Ent/Rear Climate/iPod/Roof Rails/Running Boards