If the heater core is leaking you can usually smell it. You might also want to check the drains below the cores because if the condensation on the evaporator isn't draining it will just recirculate into the car.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold