1984 Mercury Grand Marquis heater core question

1984 Mercury Grand Marquis mileage: 60,000. My 72 year-old dad has tried for two weeks to remove the heater core from this car. He has taken everything loose that he thought would do it, but is now stumped. He says about all that is left is the rivets! He's pretty sharp and this is the first time I have seen something really stump him! He seems to think maybe there is something he's not seeing that needs to come loose.