What is the voltage range for the signal wire triggering the blower motor on an 06 silverado with auto control?

My blower motor is inoperative and I would like to troubleshoot completely before ordering any parts and possibly wasting money. One day the blower motor stayed on after the ignition was turned off then over the course of the next week I lost the high speed fan and now I have no fan operation at all. I have checked the fuses and I have 12 volts down to the module and a good ground, my only concern before replacing the module is the voltage on the signal wire coming from the head unit. How does the head unit control the fan speed? I am under the assumption that it is controlled by the ground as opposed to the power because of the 12 volt constant from the under hood fuse block. Does the signal wire also work off of a ground? How can I test to make sure that the head unit is functioning properly?