[Loekee75]

Bypass your heater hose. (Hope you don't live in an especially cold area!)
That's the easiest, quickest makeshift repair you can do for a cracked heater core. Your level isn't changing because the core is just seeping...hence the steam.
If you don't know how to bypass the heater, all you do is disconnect the hose from the firewall (either hose), completely remove the OTHER hose, and connect the first hose it to the inlet in the engine, so the water will simply travel through the engine.