heater controls

Hi, can anyone advise me on a problem I have with my van heater. I have a merc vito 108d 1998. The blower control works as does the temp control,
but the vent direction control doesnt direct the air to the windscreen or side vents. The only vents that appear to work are the centre ones.Doesnt matter where you turn the control knob to, nothing happens. Is there a seperate fuse or relay unit and any ideas how I can fix this problem
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