Try this:

With the engine running, turn the temp control to full cold. Turn the mode control to full defrost. Put the fan control to anything but off. Press and hold the rear defrost button until you hear a ding. Let it go and allow the system to perform some gyrations. That alone may fix the problem. If not, it may give you some error codes in the odometer space. Note down any code(s) and come back.

 

 

ac00 means that the computer did not detect anything wrong with the system. What exactly was the temp setting when you experienced cold from the floor and heat from the defroster? It the temp setting was low, you might get warmer air from the defroster than from the floor outlets. You don't want to be blowing cold air onto a windshield if you are trying to defrost/defog it. Also, make sure your engine coolant is up to the proper level.

 

 

How much of a temp difference between the 2 is there?

Have you ever flushed the heater matrix out on the car? It could be blocked up, which could cause low heat output.

What coolant are you running and when was it last changed?