Quoted from rwenzing
You blower motor resistor block could be open circuited. The highest speed bypasses the resistors.
Unplug the 4 wire plug to the resistor block (under the HVAC blower motor in front of the pass side firewall). Then measure resistance across any two pins of the block, not the connector. There should be finite, measurable resistance between any pair of pins. Try several combination's. If the resistance on any or all combination's is maxed out on your highest analog scale or reads OL on a DVM, the resistor block has an open circuit or circuits and it's no good.
The blower can still work on the switch's highest speed because it bypasses the resistor block completely in that mode.
you should be able to find a range on your multimeter that gives a numerical value of the measured resistance other than close to zero or over scale (OL on a DVM).
This is just a final check you can make before you decide to purchase a new resistor block. it is a common failure on this part.
Look under the hood on the front of the firewall on the passenger side and find the plug in this picture:
You can get a resistor block assembly at a Ford dealer or parts house. $16~20 list.