The humidity sensor advises the air conditioning system of cabin conditions in order to keep the screen from misting. There is also one in the air intake. I have an FR which has the dual zone climate control so can't say for sure that this is present on the S or SE. You can easily tell if the wires are there. They will run in a little trim from the headlining to immediately behind the mirror. The additional cable for auto dimming mirror did not appear in my car not that I'm bothered about that.
You need a new sensor part number 5Q0 955 547 and a new light switch with auto position. Loads of the latter on eBay but ensure they are suitable for the golf mk7. They changed the way the fog light contacts worked from the
Mk2 Leon and Mk6 Golf. Doubt youll get a Seat one without paying real money at the Stealers. The golf ones are white light rather than red. I picked a new sensor from eBay quite cheaply. Careful with second hand ones that the seller hasn't confused humidity only with light function as they are very similar.
The sensor can be changed by splitting the trim behind the mirror and removing it. You'll see a join level with the ball joint on it. Once the two halves are off, the mirror will come off its mount by turning it anti clockwise about a quarter turn. You'll then see the humidity sensor immediately behind this, held in with a clip. I decided to also remoice the gel pad behind this and replace it with a new one as I wasn't sure if it was OK for the job. It may well be OK. I dampened a new one to eliminate air and placed the new one with the gel sandwiched in the middle.
You then need to code the sensor to 00095D and disregard the VCDS long
coding assistant as it doesn't quite understand this yet. Then tell the Central Electrics Module it has a rasin sensor / light sensor and that it has a light switch with auto position. Then its a case of doing the adaptations to enable auto lighting and activating the rain function of the wipers. I'm afraid I started a duplicate thread when asking for information. If you look for my other thread you'll see the comments from Dean0 which confirmed the right adaptations. Many thanks to him for that.
This is a real easy mod that doesn't involve any wiring. Cost me 50 quid and now I've got auto lights and wipers.