The air conditioner is leaking back into the floor board

There is condensation that occurs when the warm, humid air passes over the evaporator in the system. This is the part that produces the cold air inside by removing the heat through a process of evaporation. The water collected is allowed to run out of the assembly through a drain that protrudes through the firewall of the vehicle. I believe this is plugged. Look for hole on the firewall insulating material that has a small, black tube coming out of it. It won't be real long, just enough to stick out past the firewall. Use a short piece of coat hanger, drill bit, etc... to clean it out. DO NOT push anything in there more that 2-3 inches to avoid damage to the heater core or evaporator. If it's too plugged, the dash will have to come apart. I hope this works for you.