Check for vacuum leaks under the hood. The vent contols need vacuum to operate.
In the truck!
Under the hood, passenger side firewall, not far from the blower motor.
Heater core leaking?
under the dash on the passenger side.
It is under the dash where the passagers feet go.
In the fan box behind the glove box.
Yes, located in the blower motor resistor pack. Follow the wires backward from the blower motor to find it.
how to install a blower motor for a 1994 Toyota pickup
how to install a blower motor for a 1994 Toyota pickup
There could be two separate problems. Check the blower motor resister, and the ABS sensors for problems that could cause this.
where is the blower motor on a 2001 chevy blazer
The blower motor relay only operates the high speed function. That relay would be in the fuse/relay center under the hood.