99 Grand Caravan - The front AC works great, but the rear air always blows warm/hot air. Does anyone know what causes this and/or how to fix it?

When the rear system is blowing warm air, is it coming out of the roof vents or coming out of the floor vent (passenger side panel, at floor level, between middle and rear seat)?

The rear air system in my 1998 Town & Country works as follows:

When the front dashboard fan switch is in the "off" position, the system is off.

When you turn on the front dashboard fan switch, the rear dashboard fan switch is in any position other than "off" and the temperature control slides are both at full cold (all the way to the left), the controller for the A/C system will turn on the roof vent blower and blows cold air out of the roof vents. I call this "refrigeration mode" for the rear system.

When you turn on the front dashboard fan switch, the rear dashboard fan switch is in any position other than "off" and the temperature control slides are both at full hot (all the way to the right) or anywhere in between, the controller for the A/C system will turn on the floor vent blower and blow hot air out of the floor vent. I call this "heat mode" for the rear system.

The rear system will stay in whatever mode it started in when the fan switch was turned on (or the car was started) no matter what temperature you select from the temperature slides after the system is running. The only way I could find to reset the system is to turn the front dashboard fan switch to "off", manually set the temperature slides (full left for refrigeration mode or full right for heat mode), then turn the fan switch back on again.

At least in my van, there is no way to adjust the temperature of the air discharged by the rear system, it is either full cold from the roof vents or full hot from the floor vent.

Now try this: With the van turned off, slide your temperature controls to full cold, start the van, turn on the A/C and see what comes out of the roof vents. If roof vents blow warm air after this, there may be some type of blockage to the rear air evaporator or a problem with the rear roof vent ducting.

Anyway, good luck and let us know what you find.