Climate control problem
 

I have a '99 deVille de Elegance. The short of it: the driver's side vents blow hot, the passenger's side blows cold, at the same time. I'm referring to the dashboard vents. Playing with the control on the passenger armreast doesn't help. I tried looking at the vacuum lines/motors under the dash, and everything seemed to work. my guess is there's a stuck shutter somewhere in the system.
Advice/comments/questions appreciated.