I am not sure of the relationship, or if this helps, but I had a similar situation where the rear passengers were getting cold air blasted on them, even after sliding the overhead thermostat to bake. Finally we just turned off the rear air button on the dash to tolerate the rest of the trip. When I returned to figure this out, I saw that the AC button on the dash was lit (automatically), and when I turned it off, the air from the overhead rear vents warmed considerably.
Not a scientific approach, but I think that the rear thermostat is not as advanced as the front - meaning that the dashboard balances heat and cool to reach the desired temp from the same set of selected vents, whereas the rear is mixing cold from above and heat from below to reach the temp. By turning off the AC compressor, it becomes all heat. This was about 2 months ago so I will try it again tonight and see what happens.