I've got a 1994 Toyota truck. My previous 2 1994s I could use the fresh air selector to bring in air without using the fan. This truck however, nothing. I have to run the fan. If I run the fan on low, I get airflow. So that would tell me that there's an obstruction between the fan and the vent at the windshield right? Well from the hood vent, I can see down into the squirrel cage fan below. So what the hecks going on? Flapper door does work. I'm really stumped here

 

So you get air through the vents with the fan on but not with fan off while driving? The only time I ever experienced that was when we put one of those old school 80's bug deflectors on the front edge of the hood. The ones that were basically a piece of plexiglass in an aluminum channel. Supposed to deflect bugs and debris up away from windshield. Also stopped airflow to the cowl vents. Took it off right away.

If the cowl is open and you can see the squirrel cage the next possible obstruction is the evaporator. Then the heater core. If you get ANY air flow when you turn your fan on they must not be your problem. Can you change where the air comes out with the heater controls? The flaps inside should just direct output, not input from the cowl other than the one at the squirrel cage.

 

I knew it the instant I read you. Talk about a big dummy! After 10 years of running fan in the winter, I've got the biggest smile of delight today. Thank you brother! I PS...suck at the computers. I did first photo then the second and just to screw with my head, the photos flipped order.

 

It came off in the parking lot the instant I read your message

 

My wife had an 86 4Runner that had 11 windshields replaced one year. AZ is bad for stones thrown up so I tried one of those deflectors. Very popular back then. It not only cut off the air flow to the vent, it killed the fuel economy. It DID reduce ˟˟˟˟˟ hitting the glass though. And the ONLY reason I would have known it might be your problem. Been there, done it.