Antifreeze through heater?
 

1992 SE 290,000 miles.....Hi, I had been noticing for a while the antifreeze resevoir emptys quickly. I don't notice a leak anywhere. I was told maybe a radiator cap allowing it to seep out. On my way to work today the windows kept fogging up, I just drove and kept wiping for an hour! I noticed the smell of antifreeze. When I got out I could see a mist in the car. On my way home it was light out so I could see the mist coming out of the heater vents. I was told at work that it was the heater core. I turned the air down to the coldest setting and it pretty well stopped. It was a balmy 24 degrees for a 52 mile drive. Brrrr. I was covering up with my daughters baby blanket! Anyways, is the heater core the problem? If so where do I locate and how do I replace. Thanks.

That's a pretty overt sign of a broken heater core. These things happen. Your heater core is between the glove box and the maxifuse/relay center under the hood. Just drain your coolant down about halfway, get in there, and replace the heater core. Wipe up, reassemble, be warm, get to 300,000 miles.

I think there's an actual how-to on the site somewhere, but I can't seem to find it. Someone will chime in with more information pretty soon, I'm sure.

I guess that someone is me. It wasn't called "heater core," so I missed it.
https://www.gmforum.com/normal-maintainence-138/evap-clean-278997/

This will get you to the heater core. If I remember correctly, it's connected to coolant hoses by a couple of clamps, and some screws holding it in place.

TO get at the heater core you need to disassemble the dash board