I have a 1999 Chevrolet that recently exhibited a problem with the heating and a/c system. Basically, the fan has four speed settings and for some reason, it only turns on when set to the two highest speeds. If I try setting it to the two lowest levels, the fan doesn't turn on at all. Is there some sort of sensor or switch that controls the fan and is it something I would be able to replace or would I need to take it to the pros? Is it possible that it's a fuse that's bad? Any advice is appreciated.