1997 Nissan Sentra overflow drain

How do I clear a overflow drain, it leak's water all over the passenger floor when ac is on. I tried to pull blower box out from behind glove box but it hanges up under dash with what I can see all mounting screws and bolts out. I dont want to take out upper dash out too. I put up on ramps and went under car to see if there was blockage at the tube on firewall and it seems clear. What can I do to fix this problem.