if your blower motor runs with the pushbutton in the "O", off position, then the blower regulator is bad. On the 124 chassis (300E) the blower regulator is mounted underneath the blower motor, which is mounted underneath the wiper linkage. I have seen alot of both the motor and the regulator fail, and there is no way to diagnose until you pull it all of the way apart. A seized or very stiff motor will cause the regulator to go bad. Seen it many times.