Check in the basement for the location of the main drain stack.
Go up in the attic, and find the vent associated with that stack.
You might find that you can work a plumbing snake or wire "fish
tape" down alongside the stack into the basement. Pull a string
back up with the snake, and pull the wire up with that.

Good tip! I will go looking for the main drain stack. I don't even
know where it is in my house. This is about time to find out.

Jay Chan