Jakarta Expat said:
Too be more exact the sensors are built in to the back of the rearview mirror.
As for these things tripping irregularly, I have had it a couple of times, seems scattered shade/sun emitted through trees sets mine off as we have lot of old big over hanging trees along some parts of south Jakarta's roads and when that SE Asia sun get's fired up it sets it off but maybe 5 times since I have had the car the last 7 months. As well keeping the area clean where the sensors are helps as well. Other than that mine does not give me of a problem and I leave them on auto all the time.
I was not accurate but this one could be misunderstood too. The sensor is attached directly to the windshield, the device is not part of the rear view mirror but it is under the cover, on top of the windshield, behind the rear view mirror (which Jakart Expat must have meant).
As previously explained, the sensor emits light (IR) and detects the reflected light from water or rain drops on the glass (opposite side of the glass).
I have not seen wipers come on on their own. I would guess the reason still being dirt or something similar on the glass, if light conditions (IR specifically) change and there is some dirt, the detector may interpret new rain drops appearing on the glass.
I consider it a safety issue to leave the wipers on, at least at certain conditions. The point being that even if it is not raining, you may get splashed from a pool of water, by some fellow driver and get blinded by a lot of water. The auto wipers react a lot faster than the driver itself.
An -08 E must have auto wipers, I assume it is standard on all MB cars, likely including A-class.
The W221 has two sensitivity levels. I don't think that is really necessary. I remember a BMW (5-series) having auto wipers with a non-discrete sensitivity setting, not bad but not really necessary either. I still consider a clean glass and good wiper blades all that is needed, unless of course the control unit or the sensor is broken.