Depending on which sensor you manage to pickup, I know that VW do a repair kit for the sensors which is a new front plate with the special 'gel' in place ready to be mated to a windscreen.
I had my 'normal' windscreen replaced with the one with the rain sensor mounting by turbo windscreens, the chap asked me on the phone if it had the sensor or not, i said no, but i would be fitting the sensor myself, he said you'll have to have the right rear view mirror ready to go on otherwise we can't do it....
so bought the mirror cheap off ebay and had it ready, a few days later picked up the sensor off ebay for 30quid and i now have auto lights/wipers :)
the wiring is a bit of a 'pain', i managed to get the mirror and sensor loom out of a GTI that was the local salvage yard so then it was just a case of extending the wires down to the CECM and fuse box