I want to give you the findings of the cause of the continuing problem with the heater controls on your Focus.  When Client picked it up the last time, I explained to her that there was resistance coming from the heater box blend doors  when the knobs were operated.

I order to manually operate the the blend doors the dash needed to be removed.  What we found is exactly what I suspected,  the blend doors are getting hung up on the foam that seals the door when they are closed.

The heater box is a sealed unit and must be replaced as a new assembly.  One of the attached pictures illustrate what must be removed to obtain access to the heater control box.  The other picture is the actual heater box that needs to be replaced.

At this point if you do not wish to repair the defective part, I will have my Technician put your car back together, at no charge to you.  This procedure was necessary to confirm the cause of the continuing problem of the knobs breaking. Unfortunately, I can no longer  warranty the cable assemblies, unless the heater box is replaced.

The heater box includes a new heater core and evaporator.  Should either one of these components fail at a later date,  the dash would need removed again to replace them.   All in all it is not only going to fix the control knobs, but you will get two other vital components replaced at the same time.