It is on the rear passenger side, all the way in the back. You have to take out the seats and take off the trim panes to get to it. Here is a related "how to" courtesy of Autozone.com. It's how to get access to the rear blower motor, to which the resistor is attached
http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c15280041daa
If it has separate blower speed controls for the front and rear, it has separate resistors. Blower speed resistors are mounted close to the blower motor it controls for cooling purposes.
Heater blower resistor is what controls the low medium high speeds...
The front blower resistor on a 98 Dodge Caravan is replaced by removing the cover and disconnecting the wiring harness. The resistor can then be pulled and a new one put in its place.
Your truck has both. the resistor is between the switch & blower.thje resistor controls the blower speed
Resistor is a separate unit from the blower motor
Blower fan resistor controls electric current to the fan motor to maintain different speeds.
Somewhere in the path of the air that the rear blower motor blows
The blower is controlled by the blower switch, blower resistor, and relevant power relay.
Failed blower motor, resistor or switch.
Behind the glove box.
Common causes, blower motor resistor, blower motor, wiring or controls
The resistor block or blower motor resistor is used to control the speed of the blower motor. That is how you get the lower speeds on your air conditioning or heater. The higher the resistance the lower the speed of the motor.