If it is the little servo to the left of the passenger's footwell, this is how I rebuilt mine in 30 minutes with a razor and super glue...
http://www.toyota-4runner.org/3rd-gen-t4rs/183759-blend-door-actuator-stuck.html
I just finished fixing this.
Take the servo actuator unit out of the car
Mark arm position then remove it
Dissasemble plastic housing.
Find the big cog and look for the missing 10 teeth. (The only 10 teeth it ever uses)
**Mark cog position and remove it.
Shave off the copper fork contacts...
Rebuilt my heat blend servo actuator thing... Anytime it needed to close off heat it would click endlessly and hopelessly. I took it off and opened it up and found the final and largest gear was totally stripped. In the only 45° section it ever used. So I switched the little copper contact forks...