When i did mine i took out the rear lights and drilled a hole there. I was then able to spray waxoil into this area ,hopefully preventing this.Water and moisture will get anywhere and everywhere unfortunately.
I ended up cutting most of the rear quarters and was very suprised how bad these copen get with rust. Good thing now though is that she is now FULL rust proof. I have now managed to spray every part on that car that cannot be seen so she will last me nicely now while i take my time to fit all wide arches and and custom making my skirts from scratch. Next after that want to put a Subaru or Mits Evo custom spoiler on her and give it the aggressive look without going over the top. Love it as it keeps me busy and havn't built a custom car since my MK5b 2 litre Cosworth Cabriolet
My experience with the Copen is that it slowly rusts from the inside out. I was shocked after cutting into the car to find rust on the inside so when treating the car to some rust proof, don't cut any corners.
