This is not uncommon if the car is outside it could well just be water from outside, where the scuttle panel is pour some water down there, it should come straight out under car but if you have a hole it will leak into the footwell.
The heater is basically a mirror of the coolant system and has a valve to allow hot/cold and a fan (Blower) in the matrix that send the air down ducting that comes out the air vents. You divert the air the appropiate way via another control.
If it is heater it will be antifreeze and you can see/smell it, assuming you have anti freeze if not you will soon be running into problems.
Martin