Defogger problems of the 2004 Hyundai Sonata

The ac/heating electric [fan] control comes on when it wants to but will not activate when turned on. When taken to a mechanic on 3 occasions, it activates and remains on for days then stops and will not come on again. Mechanics do not want to work on it unless it does not work at all. Everyone wants to "investigate" and charge me accordingly but no one wants to tell me the actual problem. This had been going on for many months on my car, as well as other Hyundai models. The unresolved situation remains dangerous because of window fogging and lack of air flow through the vent system in the front and rear. Last winter a partially fogged window, while the fan was running, contributed to an accident during a very heavy rain storm.