How do you change the heater core in a Pontiac Parisienne?

Remove the heater hoses (after draining some antifreeze) and take the cover off that whole plastic assembly under the hood. It's a large cover that comes right up to the cowl and close to the wipers. You have to remove the long hood seal that stretches across the firewall left to right.

Now, there are many small screws (8mm head I believe and probably more than one size, 1/4" too I think) around the hood hinge and I believe one screw underneath the blower motor, so that has to be removed as well. Make sure you get all the screws and don't try to pry harshly. The plastic will break easily.

The unit is sealed and you must carefully break the seal all the way around the cover. After you have that removed you have to take another screw out on top of the heater core- it's a bracket that hold it down and pull the heater core carefully straight up- it will be a bit difficult but you must pull it out of the clip at the bottom. Wiggling back and forth may help a bit.

Now, install the new heater core by reversing the procedure, being careful not to break the plastic cover. Re-seal the cover with black silicone and use enough to be sure it will seal well. Re-attatch the heater hoses, install the blower motor, reconnect any wiring, fill the radiator with coolant and there you go!