No defrost or vent

Air or electric dash? The electric dashes are known for breaking door flaps, pull off the actuator and manual move the door. If it's broken, change the actuator motor as well. The actuators are pressure sensitive and stop when there is resistance, when they go bad they twist too much and eventually break the flapper.

 

It's been a while since I messed with an air dash, but there is a flapper that shuts the floor off. I think it was in the center lower part of the heater box. That 1 has to be closed to get any air to the dash vents or defrost, there is a second one that changes from defrost to dash vents once the floor one closes.

If I remember right, you can take off the lower right side duct pretty easily with a few screws and it gives you access to see that flap and make sure it's moving. I remember a few winter road calls where I pulled the lower duct and wedged something in that flap to give the driver defrost to get home. Changing the flap itself is quite a job, hopefully you just need an air actuator or something simple.

 

I am having a similar problem but with a 780 Volvo, air come thru all the vents at once, floor, windshield and face all together don't matter the position of the levers, what can i do to repair it?

 

I had a 1998 Century, it had air operated actuators also. The defrost stopped working, although you could still hear it doing something.
I took the dash apart and found that absolutely everything under there was plastic. The plastic broke where the actuator was mounted, so it was free to just flop around and not have a base to push/pull against to move the door.
The only thing I could do, was to use mechanics wire and tie the base end down as best I could. Worked fine for a year and and half, until I put the truck into the rocks and wrote it off.
Perhaps the Sterling has a lot of plastic under the dash too.

 

You bought a used truck. Don't expect new condition.
Either it was broke when you bought it and you knew, because you did a thorough inspection. OR it was broke when you bought it and you didn't find it because you didn't do a thorough inspection.

Or did it wear out after you bought it.
Why denigrate the previous owner?

Just making a statement... Not giving you any "cow pie".