I am going through something similar at the moment with a 85 Rx7 SE I bought last year. Car has been sitting for, I'm guessing, 10 yrs, with people trying to tinker with it here and there. I bought it knowing it would start but wouldn't stay running and the heater core needs replaced. When I took the dash out I learned every lower dash panel was cracked because someone didn't know how to remove a dash and pulled too hard...and kept pulling. I finally get the dash out and get the heater core housing out, and find out it wasn't the heater core that was leaking! This isn't the typical core that its the same lines running through the firewall, it has separate lines for that. Someone had removed the rubber lines from the engine side, and apparently had no experience with that as well. For anyone who hasn't had to remove rubber lines from the soft copper heater core lines, they get a little attached and its a pain to remove them. Most of the time you don't have enough room to try and turn the rubber line to break it free so I usually always end up cutting it off and buy new hose rather than keep pushing my luck and damaging the copper lines. Whoever removed these had their way with the copper lines and there were 3 cracks in the line! Now I get the fun job of searching for another piece which will require me finding a salvage yard that happens to still have something this old sitting in the yard and removing the dash and everything else all because someone didn't know what they were doing.