I'm not sure where to start looking, wanting to see if anyone else has had the same issue. I have a 2013 2500 6.7. I just got back from a 8hr trip up north. About an hour left in the drive, i notice that there is little air coming out of the vents. I can hear the blower motor working just fine. Just a very little bit of cold air coming from front or rear vents. I changed it to defrost, same amount of air. It is colder air, just doesn't have the velocity coming from the vents. I can hear the blower changing with changes in the fan knob.