coolant leak - heater core or more?

Hey, Ive searched a few threads and thats led me to add this. A lot of people are saying smoke in the engine bay. Well, i dont get any smoke from the engine, i get tons of smoke through my vents! I am almost 100% sure my heater core is bad, i have one, its just waiting for time to go in. Today was colder than usual and I pretty much ran it all the way to work. After an hour someone informs me my mustang is leaking in the parking lot, but it seems to have stopped leaking by the time i go out to look. So i think now its to the point where it cant really wait much longer. I have two gallons in the trunk to get me home .
So, also I was wondering what is happening. I really think its the heater core, the puddle is right under it, but why is nothing on my carpet? Not that I am complaining! But, I would think it would leak in my car some! Especially if its puddling. I am not sure if the smoke is engine overheating or the coolant burning up, it seems to be the coolant burning up, and it fogs up my windshield bad. No smoke comes from the engine. The engine does get hot and I pull over. But this is only when its low cause of too much leaking out.
I saw some people mention something under the heater core? A hole? Another tube other than the one through the firewall? What else is in there that I should be prepared for when I rip the dash out to replace the heater core? Thanks guys, hope thats all it is!

need to make two amend to my post one to reply to slow5poh.
This is a convertible. The reason I mention that is I have some extra chasis under there. In particular, I have a piece that connects from the outside of the car to the subframe up front on both sides. I have full length subframe connectors welded in my car. I cut these pieces to fit and welded them to the subframe connectors. The point is its a lot more difficult to see where the leak is coming from - especially since i have yet to jack it up and search, but it seems to be right around there, under where the heater core is abouts. So, is there some sort of hole or tube I should be trying to find underneath?
Second, this car never runs hot - unless it is low on coolant and heats up. I went to college in FL for 4 years. I put in a 160 degree Hypertech thermostat to battle the brutal FL heat and have been very pleased with it. The car used to run fairly hot, and this cooled her a** right down. Now that I am back up north this is too low, and when I do the heater core I will replace the therm to probably 180. On average now that its cooler weather the temp. gauge reads about 140. Thanks for the reply slow5poh!