Liquid by the under dash fan area

Welcome to 2CarPros.

Like you, the heater core was my first guess. However, if you are sure it isn't coolant and the level isn't going down, either you have a water leak or what you see is condensation from the AC evaporator. The evaporator is a small radiator like component which receives Freon gas at it's coldest state and is what produces the cold air in the vehicle. Condensation can collect on this and usually is collected in the evaporator housing and discharged out the bottom of the vehicle. If there is a leak, it can leak into the vehicle.

Now, I don't know where you are located. If you are in a cold climate you may be wondering why I suggested this when you aren't using the AC. It is because the AC system is used when you select defrost. The system helps dehumidify the air entering the vehicle and thus, defrosting the windshield faster.

I attached an exploded view of the system. Number 1 in the picture is the evaporator. Check to see if there is evidence that it is the cause of the leak. Also, check the drain for the evaporator housing to confirm it isn't plugged or partially plugged.

I hope this helps.

Take care,
Joe

Happy to help. Let me know what you find.

Take care,
Joe