Does it _sound_ as weak as it feels? If it also sounds weak, then I would investigate it electrically. If it sounds like it's blowing hard, esp on HIGH, but coming out weak, I would suspect a restriction somewhere.
Does it make a difference what "mode" (vent, floor, defrost) is selected? Have you particularly checked if there's significant air coming out the windshield defrost ports?
Does it make a difference what "mode" (vent, floor, defrost) is selected? Have you particularly checked if there's significant air coming out the windshield defrost ports?