Smoke coming out from vents?!
 

So yesterday I was driving and I put the temp knob for the A/C on full cold, everything was working fine and no smoke. But then I got cold and turned the temp knob up just a bit (so the arrow was between full cold and mid) and then smoke started coming out of the vents.

I went back to full cold, than to full hot, than back to full cold and nothing happens, it's only while in that small area of the temp knob that smoke starts coming out.... Any ideas?

Welcome to the forum

"Smoke" or vapor?

"Smoke" will have a acrid smell, plastic burning
Vapor will have a sweet smell, coolant

Also, I assume a Ranger truck
What year?
What engine?

2002 3.0 and I couldn't smell anything, I have a cold. But I just started it and it's not happening right now. I didn't run it long enough to get hot in case that matters.

2002 will have a Blend Door that closes and opens to expose heater core to air flow.
The warmer the temp selected the more the door opens.

And there is a Fresh air vent, it pulls in air from under cowl in front of the windshield, passenger side.

There is no "wiring" inside any of the vents, so wiring short shouldn't be the source of vent "smoke"

Vapor will also dissipate fairly quickly depending on cab air temp and humidity.

Vapor could be from a small leak in heater core, after it is warmed up.
Feel carpet on passenger side under dash for wetness.
Watch for coolant level to drop in the over flow tank.
Put a piece of tape on the side of the tank, marking its current level, then check it in a few days

If you have air conditioning then you will have an evaporator in the cab to dry the air.
It can in some instances cause vapor to form, again it depends on current air temp in cab and humidity

If it has been raining then you could also have a windshield leak, check for wet carpet, and this can leak into heater core area and be vaporized when if comes into contact with the hot core.