If you have drainage into the cabin, you may be able to splice in a new section without having to remove the headliner. Removing the headliner is not a fun job. Replacing it could be more fun that it is also.
You may be able to access/see the rear drain hose partially by removing the cargo area side panels partially and peering behind them.
My expectation is you have a wet front floor. If so, the above may apply to allow correct diversion with less disassembly. It is also not fun to try to squirm under the dash. It may be worth while to remove the front seat(s) if you are of any size.
tom