It is almost defo the famous heater resistor pack burnt out, ford did a modified pack and loom to sort this which mostly did.
The reason why full speed only works is due to it being the full 12v where as the slower speeds are lower voltage supplied via a resistor.
The resistor pack is from memory ?, on the actual motor itself, so look up above where the passengers feet are in the front behind the glove box area.
tabetha