COOLING FANs Not working

I have changed the temperature sensor three times, the thermostat twice, and now i have completely replaced the cooling fan assembly and i still cannot get the cooling fans to come on. Not with the a/c, not with normal operation. Unless i wire it up to the battery the fans will not work. Does anyone know if the blower motor resistor would have anything to do (even though all speeds work fine and a/c blows cold and heat blows hot on all speeds) i m seriously at my wits end with this car. It does not have a relay and the fuse i changed twice also.