The alternator is not keeping up with the demands from the vehicle.

The blower motor certainly demands quite a bit of power, however, not so much that the alternator cannot maintain charging voltage.

Did you run this test at idle, by chance? I might expect to see the voltage fall at idle with heavy vehicle demand.

Try the same test with the engine running at 1500 rpm or more. See if the voltage is maintained at 13.5V or so.