The blower speed regulator modulates the blower speed by increasing or decreasing the resistance in the ground leg of the blower circuit. I.e., at low speed the ground leg is floating above zero, so the motor is only seeing a percentage of full system voltage (the red wire is always at system voltage). I believe it's a simple NPN power transistor in a emitter follower circuit. Anyway, to test it regulator in the car, set fan speed to high. The red lead should show twelve volts, and if the regulator is working the blue can be checked with a test light between the lead and the positive terminal on the battery. If the light activates, the the high speed is OK. Select low speed, check again, if light is dim or lot lit, then low speed is probably OK. If you get no light on high speed, then the regulator is not conducting to ground. BTW, the yellow control voltage should not get above 8.5 volts DC.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.