91 Suinbird leak under dash -passenger side

Passenger floor carpet is soaked. Trying to get the evaporator box open under the dash. Removed all visible screws (and some not visible). Seems to be still fastened at the firewall somehow. I have tried getting underneath the car and removing any screws that penetrate from the engine compartment side. Drain on the engine compartment side is open to ground.Any ideas how to proceed?TIA

Are you sure it's the A/C drain? Not a very common problem... not impossible, but I would lean more toward heater core...