evaporator core water leak

I have a 1995 E150 that has a water leak. It leaks when the a/c is on, below the heater core. Water is coming in between the heater core case and the body. The drain for the heater core is open. Has anybody else run into this kind of problem? I'm baffled at where the leak is coming from and the evaporator sits away from and lower than the heater core.