2008 Ford Taurus X blower motor speed fluctuates high to low. Sometimes, wont run at all. Does not matter if in auto or manual temp control mode. Where is blower motor resistor located?

This vehicle doesn't have a blower motor resistor ! It has a blower motor control module , if you have automatic climate control !
If manual climate control , then there is a blower motor resistor assembly !
Blower Motor Control Processor The blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the blower motor. The HVAC control module provides a low side pulse width modulated signal to the blower motor control processor over the blower motor speed control circuit. As the requested blower speed increases, the HVAC control module increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the HVAC control module decreases the amount of time that the signal is modulated to ground.
Air Speed - Front Control
The blower motor forces outside air into the vehicle's interior. The vehicle operator determines the blower motor's speed when the driver places the blower switch in one of 5 blower speeds. The blower motor will always operate in any switch position other than OFF, as long as the ignition switch is in the RUN position. The blower motor and mode switches are located within the HVAC control module. The blower motor OFF input is connected in series with the HVAC control module by the off blower motor control circuit.
Depending upon the selected speed, power is provided to the blower motor from either the ignition 3 voltage or battery positive voltage circuits from the fuse block. The battery positive voltage circuit only provides power when the High blower switch position is selected. Power and ground are provided to the HVAC control module by the ignition 3 voltage and the ground circuits.
Low Blower Speed When the Low 1 blower speed is selected, the HVAC control module applies voltage to the blower motor resistor assembly through the low blower motor control circuit. Voltage is divided between 4 series resistors, a blower relay, and the blower motor to achieve the desired blower speed. The blower motor is grounded through the ground circuit.
Medium Blower Speeds When the Medium 1 blower speed is selected, the HVAC control module applies voltage to the blower motor resistor assembly through the medium 1 blower motor control circuit. Voltage is divided between 3 series resistors, a blower relay, and the blower motor to achieve the desired blower speed. The blower motor is grounded through the ground circuit.
When the Medium 2 blower speed is selected, the HVAC control module applies voltage to the blower motor resistor assembly through the medium 2 blower motor control circuit. Voltage is divided between 2 series resistors, a blower relay, and the blower motor to achieve the desired blower speed. The blower motor is grounded through the ground circuit.
When the Medium 3 blower speed is selected, the HVAC control module applies voltage to the blower motor resistor assembly through the medium 3 blower motor control circuit. Voltage is divided between a series resistor, a blower relay, and the blower motor to achieve the desired blower speed. The blower motor is grounded through the ground circuit.
High Blower Speed When the High blower speed is selected, the HVAC control module applies voltage to the blower motor resistor assembly through the high blower motor control circuit. The voltage energizes the blower relay, causing the blower motor to be connected directly to the battery positive voltage circuit. The blower motor and blower motor relay are grounded through the ground circuit.
Go on you tube an check out videos on trouble shooting blower motor circuit
Chevy Trailblazer EXT Blower Motor Won Turn Off

Here's a link to the blower motor resistor,usually they go bad causing you not to be able to use all speeds,it's located next to blower motor,two screws may hold it on.http://www.autozone.com/autozone/parts/2002-Ford-Taurus/A-C-Heater-Blower-Motor-Resistor/_/N-jhcdrZ91tag?filterByKeyWord=blower+motor+resister&fromString=search Here's a link to the blower motor.http://www.autozone.com/autozone/repairinfo/repairguide/repairGuideContent.jsp?fromSearchPage=true&pageId=0996b43f8036e0aa&partName=Removal+%26+Installation&partId=0996b43f8036e0aa

You dont say what Model Llimited) engine ( 6? 8? )or Manual or Auto temp control. but maybe check the ground wire for the blower. G200 black wire, which ends up under console. check the black wire at the motor, if it has voltage on it, the ground is bad. the power wire should have 12V

You need to replace your blower motor resistor!

blower motor resistor to replace, on the blower motor where the wires connect

check the resitor under the dashboard near fan blower motor

sounds like the sensor is becoming questionable or a hi resistance ground in this part of the blower sensor circuit

The low speeds(all of them) are distributed threw the fan speed switch to the blower motor resister.The resister is the problem.It is located very near the blower motor.It has two screws mounting it to the blower box.It has a plug in with about 5 to 7 wires.Unplug this plug to the resister.Remove the screws.Pull the resister toward you,it mounts and rest inside of the blower case,this is how it is cooled.Now when you replace the resister,replace the blower motor.Even if the bower motor is running fine, it is pulling to many amps for the resister.It will happen again soon,if you don`t replace the blower motor.If I can help you any more,please let me know.

The fan motors use a set of resistors to control fan speed. The lower speeds generate more heat in the resistors and these tend to burn out. While it is possible the switch could be the issue, I would start with the resistors. It is usually a module located somewhere near the blower motor.

Note: One of the reasons these go out, is that with time the bearings on the blower motor go bad and the motor takes more power to run. If the replaced resistors fixes the problem, but then fail too soon, look at replacing the blower motor as well.