I have a 2003 Chevy Cavalier, 4 door, 2.2 L. When I turn on the blower on the upper settings, it comes over very strong on the passanger side vent, and the right-most center vent. Almost no air comes out of the left-most center vent or the one on the driver side. I havn't tried to see if the floor vents come out evenly. 
Is this caused by a vent flap stuck somewhere? Would anyone know how to fix this?
 
open your dash up....I had the same thinhg happened to me....I thought it was a blower motor and it wasn't! So, I finally opened up my dash on found that there was a "gray factory foam" blocking the airway! solved the issue...
 
 
lol good luck with the dash...
3400 SWAP DONE. 187WHP 212TQ (14.2 @100MPH NA) 240WHP 310TQ (N2O)
 
 
I had this happen to.  You have a dead squirrel stuck in your vent.... I've seen it a million times!  ok seriously I have nothing good to add, that is all.