Anything below the windshield where the fresh air intake for the cabin is. If you can find access to an amprobe it's a good way to check how much current the fan motor is pulling (put the inductive loop around the power wire to the motor). Other option is a faulty resistor pack for the fan motor speed selector, but if that goes out you should still have full speed. Can you hear the fan working and just no air coming out? if so that could be a stuck or failed mode select door. Next option may be to go see someone with a scan tool and check for codes but that'll only work if you have auto climate control.