slightly off topic, but as you mentioned it in the original post...
what happens on the dash temp display if the passenger uses the temp up/down switch on the passenger door?
The temp display on that side
should go up/down, and the dual light should come on.
If you then press the dual button, both sides should revert to the driver's side temp and the light should go out.
Though to be fair, the light should come on when you press it, so it does sound like an issue
, just trying to track it down a bit...