The exact problem you have is that the thermal limiter on the blower resistor pack has blown. When that limiter blows, the electrical current bypasses all the resistors and gives direct voltage to the blower fan, high speed. The resistor pack is located on the blower motor housing in the engine compartment, held on with two screws. Look on the housing between the blower motor and the fender and you'll see a 6-wire electrical connector plugged into the resistor pack. I've had a few go out on me, easily replaceable. I just go to the junkyard and grab a couple, they're on every Ford truck and van out there.