I had a post regarding my issue below, it started Nov 2018, so been over a year.
smell when switch to floor vent
I am not able to describe the smell accurately as I have not smell thing like this in the past. After going through this thread, I finally gave this a try.
I took out the center vent cover which also included radio/fan buttons, I used a USB endoscope and inserted into it, from the camera I can see the center vent only links to AC coil, this explains why first try with foam cleaner did not help much.
Then I went to the button of the glove box, and removed 3 screws which took out the cover for blower motor. Blower motor is very easy to remove once exposed. I then used endoscope again, and notice this has access to AC coil on one side, and heater core on the other side.
I used Frost King AC coil foam cleaner ( $10 from Lowes.ca), then found a way to connect that to a soft tube so I can insert the tube in. I squeezed the cleaner inside, put the blower motor back, let it run for 20 seconds, then removed blower, squeezed more cleaner , then let blower run another 20 seconds. I was hoping this would help to push the foam cleaner through the vents deep.
After waiting for 10 mins, I used garden hose and insert into the opening, turned on water so small amount of water can flush coil and heater. I tired to gave more water, but the water ended flushing back, so the capacity there is small, and drain hole is too small to take lots of water.
I wiped out the excess water around blower motor, and put the blower motor back. Turn to fan, cold, low, then AC on for 5 minutes, then change to high heat for another 10 mins.
Now I can feel the air is much cleaner on the center vent, and much much better on the floor vent. There is still some smell from the floor vent only, but I would say 95% of the smell is gone.
Overall I am quite happy with the result, will try a few more times to flush more for the floor vent when weather getting better, as it's cold and snowing in Canada these days.