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I just checked the voltages with the vehcile running. I used the gray/black wire for my ground. With the connector unplugged, I have 12volts on the brown wire, 5 volts on the red wire. I also had 5 volts on the grey wire.
Once I plugged the connector back in, I still had 5 volts on the grey wire, even when changing the switch from vent to defost - the voltage on the grey wire never changed from 5 volts. (your reply said if it stayed at 2.5 volts, replace the control switch - does this still apply if it stays at 5 volts?