defrost mode will not blow air out of the center stack vents. it directs air to the windshield and side windows.
heat mode will direct heat to the floor areas...both front and rear passengers
a/c mode will direct air to all of the dash vents.
you can manually change some of the settings yourself depending on model. what you can't change is where the air in defrost mode is directed.
old american cars had a 'bi-level' mode..which directed air to both the footwell and dash areas.
hey, on 2/13, coming home later in the evening, my entire HVAC system shut down...like it lost power (well, technically, it did). i had zip. not even the blue backlit lights. nothing. wouldn't turn on. and it was cold...somewhere in the teens in temperature.
got home, shut it off, waited a minute, turned the car back on and it worked. been working since. now, i'm wondering if it will do it again or not....and of course, i'm at 59K+ miles...so getting it to 'replicate' for the dealer is off the table.