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There is a single sensor used and it's at the top, center of the windshield.
If you take a look at the top of the windshield from the inside you'll notice a cover that goes between the rear view mirror and the headliner, behind this is the rain sensor. You can remove the cover and then remove the sensor if you believe it may need to be cleaned.
Before doing so make sure that the windshield itself is clean in this area on the outside.
If cleaning doesn't help then some testing can be done with a scan tool to see what is going on, odds are the sensor is bad.