'92 F-250 heater won't switch from vent to defrost/floor
 

trying to help the father in law out here he has a 92 F-250 and you can't switch the heater control from vent to defrost or floor, it has the 3 knob hvac controls and i pulled those out of the dash and really not much do it just a few vacuum lines and electrical plugs........where should i start?