The rain sensor in your Jetta automatically regulates the frequency of your windscreen wiper according to how heavily it's raining.
The system uses an LED sensor, fitted behind your internal mirror. This detects the amount of rain on your windscreen, and adjusts the speed of your wipers accordingly. You can also choose different settings, depending on how sensitive you want the rain sensor to be. If you choose the most sensitive setting the wipers will wipe at just a few spots of rain, whilst the least sensitive setting will wait a little longer before wiping away. And you don't even have to worry about reversing. When the sensor is on and you select the reverse gear, the rear wipers will automatically wipe away any rain.