Blower Motor Runs Only In High
WARNING: This page is about a different car, the 1995 Ford Pickup, 1995 Ford F-Super Duty, 1995 Ford Cab & Chassis, and 1995 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
Check for open blower motor resistor. Check for open thermal limiter. Check blower motor switch.