Air only comes out on my defroster on my 1998 dodge ram 2500 series diesle

Does the blower motor work? If not check the blower motor--wiring circuits--fuses.

If it blows any air, defrost--floor--front of dash? Maybe a problem with actuator under the dash.

You may have a disconnected control cable, either on the back of the control assembly or at the flap. More information about the car make and year would be useful.

The most common problem is a stuck or faulty blend door under the dash.

Check taht the dash controls are moving to the selected option.Check for broken cracked vacuum lines to actuators for vents

Mode Door 1.
a.This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
<!--[if !supportLists]-->4.Mode Door 2.
a.When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver's side footwells. Failure is indicated by an inability to choose between defrost and floor.
b.This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It's still better than the dealer fix because you don't have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you'd most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It's inconvenient, but you'll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.

check your blower motor.

this is more than likely the #2 blend door it requires dash removal to replace a very expensive job if you have to take to a repair shop if you can do it yourself it will take most of the day dodge sell a repair kit for the problem

Random misfire means all cylinders are affected, so a shop may review the Freeze Frame and dtermine what is happeneing. I think all the air doors are electric on your truck, so the actuator may be the problem, but i would not guess as these sometimes are a real pain to replace.

it may be the switch itself they do go bad.Otherwise he could have pinched a hose or left one off. But it sounds like he is trying to help.

in the engine compartment on the passenger side fire wall you will see a black vacume line comeing out of firewall follow the line the first thing you come to should be a in line vacume switch replace it.this should help you