Rootripper - most likely the reason for the rain sensing function not working is the gel pad between the lens/sensor and the windshield dries out, bubbles form which eventually causes malfunction. The fix is about $15 and 30 minutes or so. Usually coincident with the rain sensing function going out is the auto headlight function also goes out. That is also fixed with sensor gel replacement.
When I helped my son replace his, this was the best video that we found:
https://www.youtube.com/watch?v=exYC...ReverseCommute
A couple of caveats:
1) Be aware that the lens and sensor are 2 different items. This may be obvious to some, but it wasn't to me. The gel pad goes between the windshield and the lens, then the sensor clips onto the lens. These clips are a bit tricky and are discussed below.
2) But first, as discussed in the video, block off the area where the old lens/sensor is with blue painters tape so when replacing the pad and sensor it is positioned properly inside the rearview mirror "shroud." The lens/sensor package is the rectangular item seen at right around 22 seconds into the video.
3) Right around 35 seconds in the above video, he is removing the sensor from the lens. Be careful with this step - it is easy to break one or more of the plastic clip mechanisms. if I remember correctly there are 4 clip mechanisms - one in each corner. Using a small screwdriver, each mechanism needs to slide out 5-6 mm to release the clip. If you don't slide it out, or if you push it beyond the detent, one can easily snap the clip. You've been warned - I think I broke 2 out of the 4. Fortunately it went back together just fine with only 2 out of the 4 clips. There's not a lot of pressure on the item - scotch tape would also probably work if you broke all 4 clips.
4) When you put the new gel/lens/sensor back into place I recommend doing it exactly as is done in the video. By this I mean gel pad to the sensor, then onto the windshield (careful to get it aligned properly and not upside down), then attach the sensor. I thought I would be smart and avoid any alignment problem by first putting the new gel pad on the lens, then attach the sensor, then press the lens/sensor unit to the windshield. Unfortunately the windshield has a bit of curve to it, and the lens/sensor is flat, causing a few bubble spots between the gel pad and the sensor. Using a lot of finger pressure, I was able to flex the lens/sensor enough to get most of the bubbles out and the sensor worked properly. But if I were doing it again I would do it exactly as he does in the video. I think his process would produce fewer bubbles.
Good luck. It's really not that hard, and it is nice to have the auto wipers and auto headlights working again.
Stephen