cab heater control problem

Hi, I have a 2007 Fiat Ducato Ace Milano and the heater control knob is not operating fully. It will only work between half cool and half hot. I have removed as much of the dash as possible and I can hear the flaps being operated but I presume it is either the control unit that has failed or the flaps are stuck. The fan operates without any problems. Either way it looks like a full dash removal and I am hesitant to do that. Anyone had a similar problem and a better solution?