2001 E320 Rain Sensor Intermittent

[Durocdog]

My Rain Sensor option is working only intermittently - not as in intermittent wipers - perhaps a better adverb would have been sporatically. Is it possible that the "sensor" is A) Blocked by debris B) Covered with dirt?

Not knowing how this system detects and reacts nor where the detector/sensor is located, if someone can provide to me the location of the device I can examine any obstruction.

Or, if you can suggest any corrective action, it is truly appreciated.