I just bought a 2000 Chevy S-10 Blazer 4.3L 2WD. The a/c blows really cold but it emits an awful odor and smoke or fog. When I turn the thermostat to a warmer setting, the smoke gets worse. I have flushed the radiator and it does not overheat. It smells a lot like antifreeze. I'm thinking it might be the heater core, but there's no visible leak inside the cab. The smoke comes out through the a/c vents. I don't claim to be good at working on vehicles, but if I can repair this myself and save the cost of having it repaired I will. However, looking in a repair manual at replacing a heater core (if that's even the problem) it seems like quite a task.
Check around the hose connections on the firewall first. See if there is any kind of leak. If none found then the heater core is leaking. It is a job to do, it tkes time but one could do it. Check the hoses first. One could be letting it leak go to the inside.