No Heat

I am really getting P.O.! I own a 91 mercury cougar and the heat is barley hot enough to even warm the car. I have changed the thermostat (OEM temp), fan clutch, coolant sensor, radiator and cap. I have ran a garden hose through the heater core and flushed it, checked the blend door and it swings properly, there is No heat control valve on this car that I can find, the system is filled to the top and vented. The car does not over heat in the summer. The temp neddle on the dash never reaches the middle, it actually moves up and down but never reaches half way. I have taken the engine driven fan off the car and drove it and the heat worked great which leads me to believe the car is not comming up to temp. (MY GUESS) What else could it be? Any ideas would be great.
Thanks
Sal

waterpump?

It could also be the heater core itself because there is garbage trapped in the baffle in the bottom of the Heater core I had the same problem in my 93 Cougar

replace the heater core lines. had the same prob in a ford ranger. one of them was collapsing under suction and restricting the flow