I had a similar issue on an 04 I had a couple years back. I read all about the air doors and didn't want to really mess with that process. I ended up removing the blower motor from under the dash and reached up inside with my hands and found the air doors broken and just flapping around in there. I pulled them both out through the opening where the blower motor was removed and then put the blower motor back in and I had heat in all vents including the defroster which I did not have before. weird but it worked so not sure what the purpose for those doors are