I tried myself and it was not easy. It locks in under the door sil and the entire right rear truck panel. The right rear trunk panel locks in under the trunk liner. So you have to remove 3 other parts just to get to the one you want. Not something I would try again. I would just take it to the dealer. Especially for a water leak. You want that documented so they can be responsible if it gets worse.
Go to the dealer. You have to document that. If you fix it yourself and then it starts again down the road after the warranty is up you're screwed. At least if they fix it and then it starts again after the warranty you have some leverage.
Just happened to me today. We had heavy rain this morning. I drove to work in it..no problems. As I leave work (no rain) upon acceleration, i hear and see water trickling down through the rear passenger seat belt panel. It wasnt a ton but not a tiny bit either. Not happy about it. Taking it to the dealer in the morning.