What can cause both radiator fans to not come on?

I have a 1988 oldsmoble cutless cirra wih same problem I have replaced the colant fan switch and the coolant fan relay but to no avail have not solved this problem however the fan on the back side of the radiator does come on when I turn the key off but the fan on the front side of the radiator never comes on any help yet??? You might need a diode wire. *You may have a blockage in the coolent system which can prevent the flow of the hot coolent to the radiator fan switch. Check the temprature of the upper and lower radiator hoses carefully by touch. If the one of the hoses is cool while the other is hot there may be a blockage.