My understanding is that the rain sensor and windshield are independent of each other. So the sensor has nothing to do with the windshield. It seems like you have to dismantle part of the dash and run the sensor wire down to the relay board. That is doable but complicated.
Try a search for more info, I was able to find this:
http://986forum.com/forums/showthread.php?t=15857&highlight=rain+sensor