vent selector not working

Got an 03 3.4L x cab. Heat and air work but when i turn the switch to go from defrost to vents to floor and so on I can hear it trying to change put it won't. It comes out the floor, the two side vents and the middle defrost vents at the same time all the time. Not sure what part is broke or if its something I can change myself. Any help is appreciated.

 

On the passenger side on the bottom of the heater unit next to the tranny hump you will see a unit with wires going into it.It has a white plastic linkage that controls your vent functions.You should be able to watch this move with key on or truck running from the passenger side floorboard as your are turning the vent location nob.

 

thanks. I'll try that