2001- Superduty heat and A/C blower problem
 

I have a problem with the controls on my dash for A/C heater and defroster. No matter where I set the control, the blower works fine but it randomly jumps from defrost to floor to dash on its own! I heard someone say it may be a moduale or a servo that is bad someehre in the airbox under the dash. Any advise on what it can be or how I can fix it or even a microfilm or diagram of where it is? Thanks! Russ