My Honda Ascot Innova was leaking inside the car and when we had the heater going the windows would fog up. We disconnected the heater and found that the leaking was the water in the radiator. Once disconnecting the heater there has been no leaking. Eventually the radiator blew so we just had a new one put in, but still have a disconnected heater, I want my heater reconnected but without any leaking.
The problem with most cars is that the heater core is so inaccessible a large part of the dash must be removed to gain access, so I would contact your local radiator shop and inquire as to the price of a heather core and a quote to install, as this job is often way above the means of the home mechanic.