Vacuum-powered switch no longer works...

[Centurion13]

I drive a 1998 Ranger, standard cab, RWD, no A/C.

There are three dials on the heater/air control.

- Leftmost dial controls the blower speed. Works fine.

- Center dial controls the heat/fresh air mix. Clockwise, more heat.

- Rightmost dial controls the distribution of the air and turns the blower on and off.

My problem is with the air distribution. I routinely adjust this knob to shift the warm air from the passenger's feet to the windows to defog/defrost in the morning and late evening. There's always a delay before I hear a muffled sound and the air changes direction. I understand the direction flappers or valves or whatever are powered by vacuum.

The trouble is this: today I shifted the knob from the windshield to the cabin and nothing happened. At all settings, the air continues to flow to the windshields.

I have no idea where to trace the vacuum hoses for this function, nor what might be wrong with the hoses all of a sudden.

Can anyone help?

Regards,

Steve