the sensor on my bonnet catch went on my old 1.6 A3 a couple of years ago. the wipers wouldn't work in manual mode, but worked fine in auto (i.e. when it was raining they came on by themselves)
it may be the case that the bonnet catch needs replacing (i don't think you can replace just the sensor - can't on the 8P anyway). this fixed the problem for me, but i DID have a "bonnet open" warning on my DIS
GTIs have auto wipers as standard right? so hopefullyyyyy when it matters your wipers will work. in the mean time try and get someone to vag-com scan it
best of luck