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.
