VW of America currently seems to be replacing heater cores in many of their cars for free, seemingly regardlesss of model, year, or mileage, even if it's not covered by a recall. All you have to do is get it diagnosed as a leaking heater core first by a VW dealer (so you will have to bring it in and incur the risk of them diagnosing it as indeed a leaking heater core), and then once the diagnosis is official, call VWoA customer service line and ask if they will cover it for "goodwill".
Sounded pretty amazing to me, but I got all costs covered to replace a 1995 Passat's heater core replaced by VW this way. There was no heater core recall for the 1995 Passat. I was thrilled I didn't have to do it myself because of all the work required to disassemble the dashboard, etc get at it (I hear when the car were built, they started with a heater core and built the rest of the car around it. :lol: )
Of course, if there is actually a recall out for your specific car for replacing the heater core, that could only make it easier. Assuming the recall is still valid, you should be able to call up a VW dealer of your choice, give them your VIN #, and have them schedule in advance you for the free recall work (which would be performed regardless of whether the old heater core is actually leaking or not, I think).