First of all, the fan is a 2 speed fan and it sounds like your Lo speed is out. There was a recall for the resistor blower on some models. There are also problems with wiring and fuse box connections on some models. I would suggest you call a Ford dealer and ask if any recalls apply on your vehicle and if they have been performed already (can look up by you VIN number).
You may want to do some searches on wiring or fuse box subjects.
