I agree, try Bar's Leaks as indicated in another of my posts.
It CANNOT harm your cooling system and anyone who suggests it can doesn't know how the stuff works. It is a vegetable fiber suspended in a water pump lubricant, basically.
Also, the standard heater core uses bayonet style heater hoses that just plug into the heater core pipes and are sealed by o rings. My idea is that some movement occurs in these joints causing seepage before the o rings re-seal. Bars Leaks seems to stop the coolant smell type of heater core seepage. No leak blocker can handle a hole big enough to leak significantly, it just can't hold the pressure.
Bottom line: try Bars Leaks as you have everything to gain and nothing to lose. If it doesn't work, try replacing the heater hoses using new o rings and white petroleum jelly (vaseline) to lube the o rings when you insert the new hoses. Only then assume you have a failed heater core. Generally, a failed heater core will dump a fiar amount of coolant into the car or onto the ground depending on where it is leaking.