2000 S80 cooling fan not coming on
 

Does anyone know what turns on the cooling fan? I have replaced literally everything-thermostat, water pump, radiator, fan itself. I keep getting boil overs because the fan just won't come on.
I replaced the fan once and it stayed on, so I hooked up a switch inside the car to turn it on and off. Then it stopped working altogether. I got a new fan and it won't come on at all, with or without the AC on. I am at my wits end. I am now suspecting a computer problem.