The heater core is sealed in the heater box and the box has drains (which typically drain AC evaporator condensation, on to the transfer case/forward output flange, conveniently), a leaking heater core is usually evidenced by the smell of evaporated coolant in the cabin/foggy residue on the glass and visible evidence of coolant at the drains. Sure it's not a valley pan/rear coolant pipe leak on the engine?