2003 GMC Sierra Air works but little to none in CAB

Hi, I believe your on the right track, but another thing is that. When the volume is low inside the vehicle, that usually indicates a clogged, evap core. Over time, leaves and debris, makes it into the cowl and then mixes with humidity and attaches to the fins of the evaporator core. Making is hard for the air to pass through the fins and create the volume of air that its supposed to. Although you could be right. I think you may have a clogged evap core and it needs to be cleaned, they can usually spray compressed air over the fins and break it up. But sometimes the humidity mixes with the debris and makes a nasty type of gunk, then you may have to use a brush along with the compressed air.