2006 ford taurus SE

My ford taurus's heater/air conditioning fan works intermittently. When I use the dial to turn it from 1-2-3-4 it will blow sometimes and other times it does not come on at all. It mostly works when I first turn the car on when I first use it for that day, but after a while it stops by itself and sometimes starts blowing again by itself. The heater and air condtioning both work but the fan only works to blow the air at certain times and I have no idea when it will blow or when it will stop on its own. I have changed the switch attached to the **** and it still does not work except intermittenly. I have also changed the fuse but it should not be the fuse ot it would not work at all. Also the rear window defroster does not work and does not even light up when you engage the switch. Please help if you can.

thanks Jim thomas