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It can't be anything else there but the heater core. You don't say what year it is. Changing the core is similar on all C3's. It will take a few hours.
If you chose to do it your self...I did...You need to remove the panel where the glove box is on the passengerside to get at the core box. Disconnect all connections to it. Cut the heater lines from under the car....pain to pull off. Remove the three bolts that hold the box (2 inside and one in the engine compartment. Lift and twist the box. Open the box, replace the core and reinstall.