How to stop a pinhole leak on my heater core....?

I just bought an '89 Mercury Sable. with a pinhole leak in the heater core. Now I called a mechanic and he said roughly 600 but i can't afford that. Is there any other way to get around paying that much to stop it from steaming up my car!? The guy that I talked to said an option was to get some... show more I just bought an '89 Mercury Sable. with a pinhole leak in the heater core. Now I called a mechanic and he said roughly 600 but i can't afford that. Is there any other way to get around paying that much to stop it from steaming up my car!?

The guy that I talked to said an option was to get some "Stop leak" and put that into the radiator, But that runs the risk of loosing my heat entirely....
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