Before I get into it, I'm not sure how great these rain sensors are to begin with, but mine wasn't working at all when I got the car.
While it was connected, it thought it was raining all the time because the wipers just kept going.
So I did some research and found out that rain sensors work based on reflected near-infrared light that's generated by two IR LEDs.
I pulled out my sensor from the windshield and looked at the two LEDs through my cell phone camera. (to make sure the sensor is on, I placed the key in the first position that allowed me to turn on the wipers (position 2).
On the camera I saw that one LED was dim and the other one almost dead.
I removed them from the sensor board and noticed they were flat at the top. I looked online for the closest match.
Found Optek OP266W and OP165W which are near-IR, and are flat at the top. They're smaller than the original LEDs in the MB rain sensor. Original: 4.75mm diameter, while OP266W is 3.05mm. I used OP266W. Costs less than a dollar at mouser dot com or digikey dot com.
Long story even longer, I replaced the two LEDs paying attention to polarity, and once I started the car, I turned the wiper switch from 0 to I.
It wiped a few times, then it started wiping intermittently.
I switched it to II, then to III, then back to 0 and then back to I. (I read somewhere that these steps allow it to recalibrate itself)
I switched it back to I. Wiped twice and then stopped. I turned it back to 0.
Luckily last night it rained, so I was able to test it.
It's working. It's not fabulous, in the sense that it doesn't wipe *exactly* when I want it to, but it does "read" the difference between light and heavy rain and adjusts the wiping interval accordingly.
No matter how hard I looked I couldn't find an exact match for the original LEDs but so far I'm happy with the ones I found.
If you want to attempt this repair on your own and need some more direction, give me a shout.
Have a great day,
Tudor