Phone up your Renault dealer and describe the problem to them. They should ask you to come in to get it inspected, do so and reiterate that you think it's because the window glass was fitted incorrectly. If they suggest that the glass had been replaced previously, deny deny deny - you don't have to prove that it wasn't, they have to prove that it was.
Provided that's been done successfully Renault UK will get in touch, offering to replace the tailgate or use an approved body shop to repair the rust (full tailgate respray).
I didn't notice the rust when I bought the car, but it was the first thing I sorted so don't worry about you not being the first owner. Also, the corrosion warranty is something like 12 years.