Odd A/C Issue in 2004 Gen-II

I have an odd issue in my 2004 Gen-II Prius. (Yes, I own one of them too. Had it since new.)

Recently, I have an odd situation with the A/C in which the right hand (passenger side) ducts are putting out cool(ish) air, but the left hand (driver's side) ducts are putting out heated air. I had complained to the dealer where I have it serviced, but alll they did was put a thermometer in a right hand duct and declared it to be fine.

SO, since I am on my own to troubleshoot this now, it might get solved. Has anyone else experienced this in their Gen-II before, and if so what was the root cause? I'd rather not drive around in a car with a disassembled dash any longer than necessary.

Bill the Engineer