The pressure sensor is often a problem area, on my std GTI its at the bottom left hand side of the rad as you look down from above. It's on one of the metal air con pipes. It needs specialist equipment to test it though, could just be worth replacing if you've got gas in the system and nothing working. Bear in mind though that the aircon probably won't be working anyway at the moment, it's way too cold and the low temp sensor will stop the compressor from coming on