Are you saying the cover on the plenum won't come off after you remove the fasteners? Once you have the plenum out, that's all you do...remove any fasteners, split the case, and remove the core. It can be stuck pretty good. Clean the plenum while apart. Go to your wife's bathroom cabinet and get the alcohol she has there for whatever those ladies use that for. Use that to thoroughly clean any antifreeze in the plenum so there is no trace left to stink up your car. Reseal the core to the plenum. If your core doesn't come with a new seal, then you can go to the parts store, Ace, Lowes, etc and get some weatherstripping with adhesive on one side that will work. You want to seal the sides of the core so it's tight in the case and so no air can escape around the sides.
I'll give you one other bit of advice on ANY heater core installation. (for what it's worth) There are some real junk cores for sale out there. But a quality core!! I buy factory cores on jobs this difficult. Then I do one more thing. I take it to a radiator shop and have them pressurize it to 20 psi and check for leaks. Nothing will piss you off more than to go through all this work, only to have your new core dump antifreeze all over the passenger floor board.
