What is the history of this car? Did the fan ever blow strong?

Does the fan motor sound like it is running full speed?

Have a look at the motor plug, perhaps it was miss wired at some point and is running backwards. ( A centrifugal blower will still move air in the same direction but maybe not as much. )

I wonder if there is a cabin air filter on this car and it is clogged.

Past that, someone would have to post a duct diagram as there is a chance the flaps are not in position causing restricted air flow.