The heater core is not the main radiator. Neither is it the transmission cooler. It is a seperate module in the car. They are placed in different areas of the vehicle depending on the make and model. If my reasoning serves me well, you should follow the air ducting from the interior air and you'll eventually hit it, though you'll have to remove the ducting and hoses to actually get to it. In many vehicles, it isn't an easy task. Check out my database to find one that'll be the right size for you.
http://forums.procooling.com/vbb/sho...&threadid=4882