Before looking to the blower motor , I would check the 3 prong plug coming through the firewall by the booster
This is power and regulator trigger voltage from the Control panel to vary the speeds . The red and blk are 12v. but the yellow should go from 2 to 7 volts for the trigger voltage to the regulator... if you do not have this , your problem is not the blower or regulator, but rather the control variable voltage feed to the regulator[ variable trigger voltage...]