I've got some info on this but since I'll have my Car only next week I haven't done the repair yet.
If it's not the drain, there are a few other things you can do.

I called the dealer and he told me to get a new sunroof for 1000$, I called an independent and he told me that I could get an used one for maz 500$, But that it could still leak... he told me that he could sandblast the rust off the metal part of the window for 200$ and that he could change some seals.

Also I've got that answer on this board:
You can manually adjust the sunroof if you open it into the tilted up position, then remove the plastic strips on each side to expose the glass bolts, where the glass meets the metal tracks. Loosening the bolts, then lowering the sunroof will allow you to adjust the sunroof position when closed. Perhaps it was just mis-adjusted, causing the leak. I've had no experience with the actual seals so I couldn't help you there. Best of luck!