it needs a exspansion valve for the rear air system its clauged and cant circulate
There is a leak in the rear lines running to the back.
"http://wiki.answers.com/Q/Why_does_the_rear_ac_in_97_chevy_venture_blow_hot_air_when_the_front_ac_blows_cool_air"
If the rear a/c runs on its own switch, check the switch, and if it works, you can check the blower motor, as the rear a/c has its own blower motor separate from the front. Should be located within the housing of the rear a/c. If the vehicle is still covered under warranty, this problem may be covered.
Most likely the rear expansion tube or valve is defective.
You will have a bad rear expansion valve located in the rear a/c unit.........
Start with flushing both heater cores. The rear one may be plugged with crud.
The most typical problem is the expansion valve for the rear is bad. another possibilty would be the evaporator in the rear is clogged up but that's not common.
Most likely this is the "Fresh Air Intake" door malfunctioning; rear AC's normally do not have the ability to draw fresh air, and normal AC system have fresh air intake that draws hot air from outside.
The rear air conditioner is a separate system. Dodge did a recall because some of the piping deteriorates and loses its charge. It sounds like you are experiencing something like that.
front climate control has two switches, front fan and rear fan control, on the rear fan control - turn the switch to the second notch usely marked rear fan control (aux - control) mode.
Somewhere in the path of the air that the rear blower motor blows
the ford 99 is a younge truck and if it does that then you need to get a new heat/air conditioner you might need to get a new truck.