In most cases the low blower speeds are powered through the fuse box and resistor pack. The high speed is different because it has a blower relay and a separate 30 amp fuse that is in a fuse holder near the top of the firewall. The power line for that fuse runs across from the junction near the brake booster, and directly over to the blower relay.
There is also a vacuum tank (black tank) under the hood next to the filter/dryer (silver tank). One vacuum line on the tank runs over to the intake to get manifold vacuum. The other line out of the vacuum tank runs over and through the firewall behind the glove box. From there it will have hard plastic vacuum line running over to the back of the heater controls. I have seen trucks where some idiot thought the HVAC vacuum tank was an emissions control, and disconnected it so the truck would "run better."
The back of the heater control has a large rubber vacuum connector for the incoming vacuum line and the lines to each of the three vacuum motors. As you move the heater control slider it will connect/disconnect vaccum to each of the motors to get heat/defrost, vent and recirculate air.
You can usually see at least two of the vacuum motors by just looking up under the dash. Its easier to see the vacuum lines if you take off the glove box door and look behind it. There is also a vertical plastic piece in the plenum behind the glove box and its only held on with one screw. If you remove it you may get a better view of the vacuum lines.
The hot/cold temp slider is cable operated and does not require vacuum. It simply moves a door in the plenum that adjusts how much air goes through the heater core vs. how much goes around it. One very common problem is that something fell in the defroster vent (pen, bolt, etc.) and ended up near the blend door. That may prevent the door going full travel to hot or cold.
Bruce