Below, I've pasted instructions for the Control Cable adjustment.
Function Selector And Temperature Selector Control Cable
To check the temperature cable adjustment, move the temperature control lever all the way to the left, then move it all the way to the right. At the extreme ends of lever travel, the door should be heard to firmly seat, indicated by a loud thumping sound, allowing either maximum or no air flow through the heater core. To check the function cable adjustment, see that the function lever will reach the detents at the far left and right of its travel. In addition, check that the air flow is correct when the function lever is moved through each detent provided in the control assembly. If cable adjustment is needed, proceed as follows:
The adjustable end should remain attached to the door cams.
Disengage the glove compartment door by squeezing its sides together. Allow the door to hang free.
Working through the glove compartment opening, remove the cable jacket from the metal attaching clip on the top of the plenum by depressing the clip tab and pulling the cable out of the clip.
To adjust the temperature control cable, set the temperature lever at COOL and hold. With the cable end attached to the temperature door cam, push gently on the cable jacket to seat the blend door. Push until resistance is felt. Reinstall the cable to the clip by pushing the cable jacket into the clip from the top until it snaps in.
To adjust the function control cable, set the function selector lever in the DEFROST detent and hold. With the cable end attached to the function cam, pull on the cam jacket until cam travel stops. Reinstall the cable to the clip by pushing the cable jacket into the clip from the top until it snaps in place.
Install the glove compartment.
Run the system blower on HIGH and actuate the levers, checking for proper adjustment.
Should the Control Cable turn out fine, then it's most likely to be the Blower Motor Resistor. It's a common Ford issue where some fan speeds fail in Ford's climate control but others work. In your case, it appears the lower speed is failing.
An illustration of the Blower Motor Resistor is linked to below.
Removal & Installation
Disconnect the negative battery cable.
Disconnect the wire connector from the resistor assembly.
Remove the 2 screws attaching the resistor assembly to the blower or evaporator case and remove the resistor.
Installation is the reverse of the removal procedure. Check the blower motor for proper operation in all blower speeds.
Best of luck with your problem!