I sofar didnīt hear of somebody who was able to unplug the heater core, but its worth a try. You did check the heater core hoses in the engine compartment close to the bulkhead that they are not hot?
You could easily disconnect these hoses and flush them with a garden hose, so that you can see if there is something plugging the lines. Most of the time its the heater core itself which is plugged. Donīt know which substance you could use to clean it.