Not a classic car issue, but a problem I had on a newer Ford Focus: The heater blower does not work in switch position 1-3, only on position 4.

The solution is very simple. The blower has a pre-resistor that adjusts the rotation speed. If the resistor is burned or in other way broken, the blower only works at maximum speed.
To fix the problem, you need to replace the pre-resistor. But...the main reason why a resistor burns is a problem with the blower engine. To keep it fixed longer than a few days you should inspect and lubricate your blower.

The Ford Focus has the blower and resistor under the glovebox. All you can see is black plastic and some wire connectors. The first picture shows the installes pre-resistor. Simply unplug the wire and loose the screw at the lower end.

In the next step you should remove the heater blower. Mine was attached by 3 screws. Newer ones have a bayonet mount. Don't forget to unplug the wire connector.

After you removed the blower, you need to remove the blower cap. It is hold by 3 rubber noses (see red arrows in the picture below). Press all noses and carefuly pull the cap. Take care of the two wires between the connector in the cap and the engine. On my blower engine corrosion was visible at the axle of the engine (see green circle). I lubricated the axle endpoint with some WD40 and the unit turned smooth and without noise.