I can't add to the other suggestions, but as you probably know, antifreeze is quite toxic, and you don't want to be breathing the stuff! I wouldn't suggest using the car until you can either fix it or drain the coolant and replace it with plain water (i.e., no chance of freezing).
--
'81 GLT 245 W/245K; '83 DL 175K

If you are really lucky, the leak will be from the hose connection(s) where they tie to the heater core. If you can remove the little central storage compartment in your upper dash panel, or the glovebox, you can get a look at them. Look for whitish powdery deposits at the joints.

Otherwise the leak is in the core itself. As said it's DEEPLY buried in behind the centre console and requires removal of a very large pile of parts and disconnecting a lot of wiring to access. I just replaced the one in my son's 81 and it wasn't a job I'd recommend to a rookie home mechanic. Took me many many hours and the car was out of commission for a long time. You may hear from others here that they have done the job in a single day, but I have to believe that's with previous experience or with excellent skill level.

Take a look at Art Benstein's page regarding blower motor R&R. The heater core is another layer of parts and difficulty beyond this.

go to: http://www.cleanflametrap.com/

With that observation, I'd say even a "rookie" can do this job, but be prepared for a lengthy downtime, and a lot of questions directed back here...which we'll be glad to help with. Your motivation? this job is probably booked at about 6+ hours shop labour by a professional at, what, $70/hr?

--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar 77 MGB, and a few old motorcycles)

It would seem that the heater core is leaking coolant. It probably only leaks and therefor steams when the car is running, making it difficult to detect. That problem would be consistent with the symptoms you describe.