A/C Vent Problem - 2003 R3500

I am having a problem with my dash vents on my R3500 Cummins Diesel.
As you sit in the driver's seat, there are two dash vents for the driver. From the passenger's seat, there are three dash vents. When I run my A/C, I
get ice cold air out of my driver's dash vents; however, outside temperature air is blowing out of the passenger's side.
I'm assuming there is a switch or acuator (sp) that is not working on the passenger's side.
Does anyone have any idea? Is it easy to get to? Where do I locate the switch?
Thanks in advance. Bob

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