See the useful technical thread index at top of this forum. It's not hard to change the heater core, just time consuming. Basically you have to remove the center console, drop or move the dash, drop the steering column to access the heater core box in behind on the passenger side. The box which holds the core is held in place by some nuts and bolts, some accessed under the hood. Also you can do a search in the forums, there are plenty of threads on this. Give yourself at least 3 hours (most likely 4 to 5) if you're doing it the first time.