I have had the same issue for sometime. Every time that I turned on the heater I would smell antifreeze. When the carpet started showing stains is when I put two and two together. I called Merlin at St Charles Isuzu and he said that the core was a bad design to begin with and that they don't make them anymore. What they do he said is sell an entire new "box" of a different design to include linkage and all of the controls. It was around $350.00 I want to say. For now I'll just wear my 900 down TNF Jacket to stay warm.

1995 Honda Passport;
Some people wait a life time for their ship to sail in, while others simple grab the oars and begin paddling
Yours truely
http://www.zutah.com