The front temp blend door is malfunctioning, probably a faulty actuator.
The front expansion valve is clogged.
There are separate expansion valves in the rear, so even if the front system is failing, the rear unit could still work normally.
There is no heater matrix for the back air vents, only and evaporator for the A/C. My suburban always blowed cold until I added an extra rear heater matrix.
warmer than a cold front and colder than a cold front
Try replacing your rear themostatic expansion valve. If it is plugged, refrigerant won't flow to the rear evaporator.
Possible blockage in the rear air units expansion valve. I just replaced mine for the same reason.
A cold front occurs when a cold air and a cold air mass hits each other and the warm air rises
You can replace the air filter with a cold air induction.
A cold front brings in cold air. The cold air causes warm air to rise quickly. The rising air forms cumulus clouds. There is often heavy precipitation at a cold front.
A cold front forms when a cold air mass moves into and displaces a warm air mass. As the dense cold air pushes the warm air up, it creates a boundary known as a cold front. This usually leads to showers and thunderstorms along the front.
Most likely the rear expansion tube or valve is defective.
1) Warm front - warm air mass replacing a cold air mass at ground level. Typically shifts wind southeasterly to southwesterly. 2) Cold front - Cold air replacing warm air at ground level. Tyoically shifts southwesterly to northwesterly 3) Stationary front - Equal amount of energy between warm and cold air masses creating a "stalemate".