Took my truck out for a drive yesterday, and noticed that I had no air coming through the vent while the fan was on the lowest speed. Well, when the truck was moving air would come through the vents from the truck moving, but while sitting at a stop no air is coming through. Anything I can check or do I need a new switch?

 

Same thing happened on my buddy's Chevy... turned out it was a resister under the passenger's feet.

 

It will be either the switch or the resistor (which is in the plenum on the passenger side)

 

Fan (and all other functions for that matter) operate fine on all other speeds. Any way to check the switch and/or resistor? Is the resistor on the same place on an F-150??

 

Pull the resistor out and inspect it. It's easily accessible from the passenger side foot well (this ain't a Chevy and it ain't under the seat!).

Check for charred or melted connectors while you're at it.

 

This is what it looks like out of a 150