92 T-Bird----Heater Core Replacement---help needed

I have been working for three days on trying to replace a broken heater core in a 92 T-Bird. I have removed much of the dash and center console to gain access to the core itself. I have already got the coolant tubes removed off the ends too. Now the major problem I am having is the fact that the core will not just pop out for me. In the diagrams of the ford and chilton's books it shows nothing holding it in, but the core is not wanting to come out. Is there a nut or bolt that is holding it in place? There is a big piece of rubber sealing the firewall...Is that what is holding it in place? I am looking for some help in solving this. Any advice would be helpfull. Thanks, Darrenoid