Did this on a rear light cluster of my first Golf GTi, no idea how it got in as the cluster was a sealed unit but the hole did the trick.
I had similar problem with a front indicator on my mog and used some caravan mastic tape that was in garage and appears to have fixed it. It is a common problem and I amaze people when I point it out to them and jokingly suggest they it is a feature Morgan add to tell if you are parked on the level (acts like a spirit level!)
