Since we finally got a real day of cold weather down her in the lonestar state i finally used my heater for the first time since I got the truck in July. For some reason it's only blowing luke warm at best and no longer switches from vent to defrost or floor, it's stuck on vent. I tore the vent door from right above the blower motor when I got the truck if that helps any.

 

There are 3 blend doors if Im not mistaken. Seems you already have had that issue before.

Make sure your coolant level is good. If it is low, the heater core will not get the proper amount of HOT coolant.

The other thing is to make sure the heater core is not clogged. You can try to check this by checking both hoses under the hood and make sure they are hot. They attach to the bottom of the water pump