As much as I hate recommending "mechanic in a can" type fixes, a bottle of Irontite might be worth a shot if the core just started leaking recently. I've heard a few guys say it seals up leaks in rad cores and heater cores fairly well if the leak is slow/fresh.
For $12 it's not the biggest risk in the world.
not recommended! If it "plugs" your heater core it will drastically reduce your "cabin heating" ability. And its not easy to undo.
I agree that it's not the recommended fix; they have already installed a new heater core (which IS the recommended fix). I was offering an option that would avoid removing the dash again.
I'd like to clarify a couple incorrect statements: Irontite will not plug up your heater core. It will not drastically reduce your "cabin heating" ability. It is incredibly easy to undo: flush the cooling system.
However, it should never be considered as more than a temporary fix until you're able to do the proper fix.
