I have Volvo 850 and the heater core is leaking... again. The same thing happened 3 or 4 years ago.
Is there some design issue that is causing this to happen repeatedly? If so, any long-term fix?
Did you use the recommended coolant fluid? Did you mix coolants of different colors? Coolant formulations differ greatly.
The engine coolant fluid has to be changed periodically (usually, 2 years for the old green one and 5 years for the long life ones), otherwise its additives degrade and they no longer perform as they were originally meant to.