I had the same issues as the others described, smoke (not mist) coming from vents. It really perplexed me. I thought, smoke, the fuses aren't burning out, fan works with no noise, etc. I thought about it and thought about it, what could be the source of this smoke. Another crazy thing that threw me off was, AC stopped working a few weeks ago (everything working fine just no freon) and the smoking stopped. I had it repaired, air working fine, smoke starts again.
Did some searches and people said to check the evaporate core drain.
I went and checked it out sure enough, I could see crud stopping it up. it was draining very slowly. Also, as for the source of the smoke. I noticed the drain rubber grommet had completely fallen out of the firewall and was lying on the heatshield ( few inches from the exhaust pipe). I found this after driving some distance. When I popped the hood and checked out the drain, I could see smoke coming from the hot rubber grommet. Turning on the air conditioner pulls this smoke right into the cabin of the vehicle.
So, I figure the source of smoke is from the grommet and it is filling the water around my evaporator core resevoir and dash with smoke particles.
The evaporator core was filling with water (alot).
It was happening really sporadically which threw me off as well.
So clean out evap core drain and put in new rubber grommet. I thought that I was going crazy.