1990 379 low air volume from vents.

You're either losing air somewhere, blower motor ain't working good, blend doors not sealing, or the thing could be completely covered in dirt.

How many times has the housing been fixed?

 

Do you feel any air loss around the box? And do yoy feel air from multiple vents? Such as floor, defrost, vent

 

Have youbreplaced the cabin air filter?

 

Pull the Heater core and evaporator. Most likely blocked by fine dirt, debris.

Clean or replace.

The filter is just a rock catcher.

 

My former 1994 379 didn’t flow enough air in high in my opinion, even after I blew air through the evaporator. The sleeper would freeze your arse off. My replacement truck which is a 2014 Pete, on the hottest day, no need to go higher than 2 on the dial out of 4.

On my 379(former), I ran a clip on oscillating fan between the seats and would circulate the air better.

 

'91 379 here. I feel your pain. I removed the glove box then a/c fan. Cleaned the fan blades well (bird cage?) and flushed out the heater core as best I could with some kind of top secret OMG Heater Core Cleaner Outer foam and standard garden hose. Lotta crap was flushed out but didnt really help much. Fan needed a good cleaning though. I do have some cracks and leaks in my dash too.

 

Truck is 33 years old. Check all the ductwork. It may be cracked, disconnected, not sealing at the joints/connections. Blow air through the ducts. Maybe a critter made a nest though most of the time that ends up in the fresh air intake and blower motor.