[Eric@StarSuzuki]

In my experience, most likely a leak from the heater core itself. Though the hoses do go bad as well. Hoses usually leak coolant on the floor, and on some cars the piping from the heater core goes through the firewall into the engine compartment and the rubber hoses are entirely outside the passenger compartment. I'm not sure about the 126 heating system, but I'd be surprised if there was rubber hoses in the passenger compartment. In any case, the rubber is never in the airstream for the HVAC system, so if you smell coolant, or if you're getting some coolant colored fog on the inside of the windshield, it's most likely the heater core itself.