That's what you pay a mechanic to figure out. We can't do that over a computer with no test results to analyze. The most common suspect used to be the motor itself, then an overheated switch, but starting with the redesigned '96 mess, the engineers found a solution where there was no problem by adding a notoriously unreliable computer to the system that never needed a computer before. They did that to allow 10 percent air flow over here, 20 percent over there, and a little up there, all at the same time. Computer circuits can't handle the heat from switching the fan motor current on and off so they had to add a power module. Those have caused a lot of trouble. Also, to make the speed continuously variable instead of simply having three or four speeds, those power modules use "pulse-width modulation", meaning they switch the current on and off about 400 times per second. They vary the on to off time to vary the speed of the motor. All that computer circuitry lives in a hot environment that electronic components hate so failure is common. That pulse-width modulation makes it difficult to get a proper reading with a digital voltmeter too. You need a less-expensive test light to test for voltage to the motor. If there's voltage and a good ground circuit, but the motor doesn't run, suspect the motor. If there's no voltage to the motor, suspect the power module, but the computer could also have a problem.
One word of warning about the computer. If you unplug it, or if you disconnect the battery, that computer has to be recalibrated, otherwise the six yellow leds in the switches will be flashing. Performing the mode door calibration is pretty straight-forward and easy, but getting the "cool-down" test to pass can be extremely frustrating. It's not uncommon for it to take 20 attempts before it passes. That test will never pass if the air conditioning system isn't working properly. I can copy the calibration procedure if you need it. It simply involves pressing the right buttons at the right times. Don't try this unless you really need it because you can end up with those flashing lights.
Friday, February 14th, 2014 AT 7:23 PM