Leaking upstairs drainpipe in downstairs wall
For years, I've had a small amount of water in the crawl space under my house. Directly above that is my wife's bathroom. We had a towel rack hung above the toilet with some very unsturdy screws. Yesterday, the towel rack fell, exposing a larger mushy hole. So I started pulling back the drywall, which was rotten. A PVC pipe is obviously leaking on the other side of the wall and has ruined the wall. It had been hidden because of the wallpaper which was painted over. Upstairs directly above that is another full bath, and I'm assuming both the toilet, bath and sink would all drain through this pipe? Correct me if I'm wrong. When I flush the toilet upstairs, obviously there is some flow down the outside of the pipe.
Here's my question.
I realize I'm going to have to replace the drywall in the downstairs bathroom. So I'll just start low and go to the ceiling pulling it off? Will I be able to access the joint or fitting that is leaking? If not, where would I go next? Into the ceiling of the downstairs bathroom or the floor of the upstairs bathroom? Any help or advice would be great.