The electric condenser fan is intended to assist in eliminating heat from the condenser coils at idle when the air conditioner is on and when the cooling system is over taxed. This helps the AC continue to cool at idle when a minimum of air is circulated through the condenser by the main belt driven thermostatic fan. The condenser fan is switched on by a thermostatic switch in the radiator when the cooling system is burdened by excessive heat . What appears to happen is, as the switch ages, its temperature threshold, the point when it switches on in response to increased heat, rises over the years. In older cars it may deteriorate to a point where it no longer or rarely turns the fan on. The switch is located on the bottom left engine side of the radiator. To see it, remove the large black plastic fan cowling and look down the left side of the radiator. You will see a switch with two wires and a connector sticking out of the radiator.
There are two ways to approach this problem. You can replace the switch or bypass it. Replacing the switch would be the right way to do it. However, if it's an older car you don't know how fragile the radiator may be. You could rip a big hole in it. If you are unsure of the condition of the radiator you can easily bypass the switch which will have no ill effect on the car. What will happen is the fan will run continuously only when the AC is on.
To do this, simply cut the wires at the switch and splice them together with a good crimp type blind or barrel connector. DO NOT just twist the wires together and put electrical tape on them. Do it right!
Nog beter is solderen!