2003 Chevy Trailblazer Blower motor Voltage check

If this is an automatic climate system, and you had voltage at both the purple wire(a) and black wire(b) then I would suggest you have a faulty blower speed processor. If it is a manual climate system, pin B should be hard wired to ground, but is junctioned within the blower resistor, so if you had volts on both I would suspect a resistor. I would assume the method you used, was either a voltmeter with one lead to ground and the other probing each wire and got voltage. Or you used a test light to ground and got a light from both circuits.