Adjusting the operation of the RegenLichtSens (RainLightSensor) is possible through Vagcom/VCDS or the VAS tools that Volkswagen dealers use however adjustments do not make a large difference in the operation of the sensor especially where you have the standard green glass.
Using Vagcom/VCDS open controller 9
Select the dropdown box and select option 2
Select config helper (may require version 10 of VCDS)
Light activation threshold can be adjusted here along with Highway light function (see Volkswagen Golf Auto Headlights above 140kph) and Rain Closing (see Volkswagen Golf Auto Rain Closing Windows and Sunroof)
Light sensor correction.
Rain sensor correction.