If your vehicle is showing blower fan only works on the highest speed setting, here are the most likely causes and the parts you'll need to fix each one.
The blower motor resistor reduces voltage to run the fan at lower speeds. The highest speed setting bypasses this resistor entirely, which is why it continues to work when the resistor fails.
High electrical resistance can cause the plastic connector plugging into the resistor to melt and burn. This breaks the electrical connection for the lower speed circuits.
As a blower motor ages, its bearings wear out and it draws too much electrical current to spin. This excessive heat will repeatedly burn out the blower motor resistor until the motor itself is replaced.
The internal electrical contacts for the lower fan speeds inside the dashboard switch can wear out or corrode over time, leaving only the high-speed contact functional.
If recent electrical work was performed, the blower motor relay or resistor wiring may have been incorrectly bypassed, forcing the fan to receive direct 12V power at all times.