1998 Chevrolet 3500

Just because the Blower works doesn't mean you will get air delivery. If the Blower is working but the Blower FAN is not spinning you will get no air delivery. Check out this video to learn more about the blower unit. https://www.youtube.com/watch?v=VpYX663B7Mc

Could be the blower motor, or a restriction in the heater box, or a blend door problem.

check your blower motor.

they damper door is stuck could be on the dash control or the damper door itself

Sounds like the "Blend Doors" are not completely closed (or opened). The Blend Doors open and close to direct the air from the blower motor to the vent selection you have made on the control panel (defrost on windshield, floor, front, etc) You may need to try various selections to see if the blend doors actually open or close. If no changes, it could be a blend door actuator failing.

If the blower motor is capable of working on all the speeds but you only get wisp of air out of the vents then the fan side of the evaporator core for the a/c may be coated with crud and debris which is restricting most of the air that the blower is moving. Usually this problem comes on fairly slowly until one day it is realized that hardly any air is coming out of any of the vents even with the blower on high speed. The blower may even sound like it is working harder than it should to create this little bit of airflow. The fix for this is to remove the blower motor and look into the plastic air plenum with a mirror or flexible borescope and check the a/c evaporator for a thin coating of gray crud. If it's present then I use compressed air and a long blow nozzle [bent to get close to the core] and blow the crud loose. You may not get it all but usually you can blow out enough of it to get back 90% of your airflow. If however your blower doesn't work on every speed then you may have other electrical issues that are preventing the blower from reaching full speed.

There is no cabin air filter for your vehicle. Do you hear the blower fan?

Your selector panel is bad, ( temp, blower, ect.) all one unit, my 95 did same thing chg. that unit and it fix it.

check for debris in the blower fan

I took mine to the dealer and they found a plastic back hand been sucked up into where the blower pulls air in by the passenger feet.. Good luck. Dealer charged $75.00 to pull it out.