1998 Cadillac Seville Not Cooling Enough

My radiator fans turn on only after the car goes above the mid way temperature point and doesnt cool off the engine often if at all depending on driving distance. Most times I have to turn on the heater to get the car to cool down and I think the cooling fans had multiple speeds but it only runs at one speed which must be 1st speed, the car will overheat whether driving on the highway or streets. Has plenty coolant, replaced thermostat twice, replaced radiator and hoses, overflow cap, Im thinking coolant fan controller but dont know where its located or whether it speeds up the fan setting automatically