Actually the blower regulator is in the ground side of the circuit. Control is measured by the voltage drop across the transistor. If the base control is appropriate .5v to 6v and the drop across the regulator control can't get as low as 1v then the regulator is defective. When the blower is at low speed the drop across the regulator is in the multiple volt situation probably 6v or so. In Hi blow the drop across the regulator is in the range of 1v.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician