I just got this truck, but air will blow out of the vents but will not get hot, only when blowing from the defroster. Can anyone help? AC works fine when on vent mode.

 

I am no automotive HVAC expert, but it actually sounds like whatever means the hot air uses to get to the vents is broken. I don't know if there is some ductwork / conduit in there that is disconnected or what. If it was a vacuum issue, you would not be able to move the flow from defrost to the vents. If it was a heater core, you would not get hot air.

But, since you have hot air and are also able to control which vents the air blows out of, the only thing that I can think of is that the hot air is not making it to the vents.

Just guessing...

 

the door inside could be seized or broken. i'm unsure of the name of the part. or it could be a vacuum leak