Does the car have AC? IF so the heater core is going to be part of that same unit. The heater core and AC unit will be stacked sideby side and connected together. You basically have to remove the whole thing. If you can figure out how the heater "black box" and the AC "black box" are connected and get it undone, you could just pull the heater. I have never been able to actually do that, but good luck.
heater cores are a pain.