So I thought I would share my mechanical experience with one of my pickups not heating...is a 2005 Ford F150 Lariat with the 5.4L V8. Just started this winter. Antifreeze level was good, thermostat was good, blend door was functioning. Most guessed heater core. I did some research online and decided that it wasn't the heater core because the only thing I have done since last winter was add a touch of antifreeze. So what I did was replace the water pump, and while I had everything off in the front I replaced the thermostat as well, the thermostat was working correctly but hey it was only $9. I drained all of the antifreeze as well. Then I put everything back together and then bled the coolant line as the truck was running. Low and behold I no longer have a problem. Truck has about 174K on it and the water pump was original. Just though I would share this little trick because the winter usually brings out the worst in our vehicles. Hope this helps someone!
Toby
Toby