Move the carpet, remove the plastic cover just to the right of the gas pedal, left side of heater core and look at the heater core compartment and 2 tubes that go from the firewall to the heater core. There are NO clamps there, they're sealed by o-rings but I've seen those o-rings leak too.
Considerering your symptoms, I'd suspect the core is leaking and it'll be obvious when you look at it. If there's any doubts, you can also have it tested with coolant pressure tester. The good news is that it can be replaced w/o removing the dash as there's 4 screws that hold it's housing to the climate systems housing. Don't let anyone tell you that the dash must be removed for THAT.