It's a fairly common ailment of these cars. I dealt with the same symptom for a long time, but eventually broke down and replaced the heater core in my 850. It's not a hard DIY job, just takes some finesse and a little bit of patience.
When I got the old HC out I couldn't detect any leaks at all, but the new one cured it. I'll also note that I replaced only the core, not the seals at the firewall. Most talk I've read here include lots of questions and hassles about the bushings/clips and such at the firewall connection. Mine wasn't leaking there so I avoided them.
-Will
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850 / Mini
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