When the weather is damp & cool and the heater is on, the windshield tends to fog up. This generally happens when the engine is warm, for example, after a short stop at a drive-thru restaurant. It almost looks like the heater blows moisture to the windshield. What might be going on here?

You could have a possible leak in your heater core, if the mositure coming from the vents smells a little sweet, that is also another sign.

Of antifreeze, Also check the passenger side floor to see if it is wet, if it is then it the heater core.