I doubt the heater core is plugged, why would the core only get plugged? A blown head gasket is more likely the issue, it allows air to enter the coolant system to the point the heater core air locks and no longer heats.
The heater in my wifes truck stopped working so I researched forums and discoverd a lot of people were replacing "plugged" heater cores in her year of truck. I removed the hoses serving the core in the engine bay and flushed water through it and there was no blockage. I then tested the cooling system for combustion gases and bingo. I replaced the head gaskets a few years ago and problem solved.