Cold in the way way back

The rear vents in my '09 Toyota Sienna minivan do not blow heat, only cold. The vents up front work just fine, but when I turn on the rear heater (whether from the front controls or from the rear controls), my kids get iced. They never blow heat, even after long periods of time- sorry about that, kids. (Also, the upper vents in the back do not blow strong, even on High, although the lower one does). I heard something about an interior heat sensor, and about blend doors, but I need help on where to start looking. Thanks!

There is a water valve servo motor that may not be working.