Air Flow Restrictions
Airflow complaints are sometimes caused by restrictions. Restrictions are generally a result of dirt accumulation. Dirt accumulation can be in the form of animal hair, dust, leaves, or even an occasional napkin or fast food receipt sucked out of a glove box. On more than one occasion I've even seen rodents take up residence near the blower motor. While this is fairly rare, the soft bedding packed into the vanes of a blower wheel by an active mouse will certainly cause some airflow problems.
The first place I look for restrictions is at the air inlet. If the inlet is blocked, the fan will not be able to pull air in. Inlet restrictions are usually caused by lighter, larger objects like tissues and paper. The source is often an overfilled glove compartment.
Can a dirty cabin air filter cause airflow problem?
Certainly. More cars have cabin air filtration than ever before. That means more potential for airflow problems caused by dirty cabin air filters. Filters are often hidden quite well. Sometimes the owner isn't even aware that his or her car has a cabin filter.
All sorts of nasty things collect on the face of a cabin air filter.
How about the heater core or evaporator core?
From my experience, dirty evaporator cores are the number one cause for low airflow. When evaporator cores start to leak, oil from inside the a/c system begins to coat portions of the coil surface. Dirt sticks to the oil, which forms an airflow dam. The entire surface of the core may become blocked with dirt. Since the underlying problem is the leaking coil, replacement of the evaporator is the only long term solution.
What about the blower motor?
I have seen plenty of blower motors wired backwards. This happens when someone replaces the blower motor and reverses polarity. You can usually tell if the motor is spinning backwards. They sound like they are working well. Too well in fact.