So I think I have a solution to this problem. I can't be sure yet, but everything is looking good so far.
But first I want you to answer this question, Do you have a cabin air filter in your car? (if you didn't install it, chances are you don't)
So my blower motor finally died. It had run enough to ware the brushes down to nothing. So I installed a new blower motor.
But first I thought it'd be a good idea to blow the dust out of the vents. Thing is there wasn't much dust that came out. But boy oh boy was there leaves, sticks and other tree debris, all in little bits.
Seems that leaves and such were chopped up by the blower motor and stuck on the back side of the heater core.
In the process of blowing all this out I rotated the vent selector back and forth. The more junk that cam out, the freer the knob got.
So it seems to me that debris is the main problem as to why things get sticky and break.
Now, I haven't had a cabin air filter in my car ever. That is 11 years. Which time wise seems to fit with others I've seen with similar problems, and no solution has been posted to my knowledge.
To prevent this, I'd say a cabin filter should be used in every car.
To get the junk out you'll have to remove the blower motor. Not difficult, it's only 1 electrical connector and 3 screws. But it's an awkward angle to work at. Stretch before wiggling under the dash so you don't cramp up.
Grab the air hose and blow all the debris out. Change the direction of the vent and blow again. Continue this until you feel you got most everything.
Should be good for another 10 years or so.
Best of luck.
Here is the file on how to remove the blower motor --> http://www.filedropper.com/focusblowermotor