There could be an obstruction
This is from a 97 SSEi. The pine needles you see accumulating are the beginings of a blocked evap core. This amount however wouldn't cause any problem:
This is the blower housing from my 90. Here you can see a rodent nest. This would cause some air blockage:
This is the evap core. This is mostly clogged with general funk, some of the rodent nest and maybe mildew. This amount of crud can definately block a good amount of air:
You can remove the Blower Control Module and inspect the blower housing for contaminants. You could also use a shop vac and slender tube to remove most of it. As far as the evap core fins go, I've heard of a foaming cleaner that can be sprayed on the core to clean and disenfect. The idea is to spray it in, let it foam up and do it'* thing and then rinse it off. Obviously, you would want to do this in a garage so you don't have an ice build up this time of year. Also, you want to make sure that the evap drain is clear before applying.
The other problem might be that although the ECC display indicates that the air should be coming out a certain vent, it might actually be coming out the defrost vents. This can happen with a loss of vacuum to the programmer. Just give that a quick check before digging into the blower housing.