No heat and it's getting cold

Here's the problem:

It is starting to get much cooler here in the mornings.

After the engine has heated up, unless she is constantly driving, the fan does not throw any heat.
If she is in stop and go traffic, nothing.
The fan works fine ... air it circulating, but there is no heat coming out of the vents.

I had the stat replaced last year and the coolant checked (it was fine).

It is a 1996 Ford Escort with the base 1.9L engine and auto tranny with only 125,000 km's on it.

Does anyone have any thoughts as to what the problem could possibly be?

I know this may sound stupid but recheck your thermostat. If she is only going short distance and the engine has not really heated up you will still get cold air. The other thing to check is that when you set the tempature in the car that it actualy switching to heat. Sometimes the gates will stick or break not allowing the hot air to enter the car. I don't have exactly the right terminology on the word gates. But it is something inside the dash not on the surface of the car.