a few ways to solve the problem.
1st. the problem lies in your fan control module. 87-89 modules are an 8 wire module deal under the dash above where the brake / clutch pedal is. Most of the time the wires are fried going to the module.
if it is a 91 to 93 the module is located in the engine compartment - on the passenger side shock tower. it is bundled with the fuel pump relay. not sure where the 90 one is, it could be either place.
you can try to fix / replace the control module and solve it that way.
you could also go to autozone and in the cooling section they have an aftermarket fan control relay deal. from memory it's about 35 bucks and they sell you a wiring harness, a relay and a probe that you place somewhere (like in the fins of the radiator). THe wiring would be a fused lead to the relay for both the fan and the relay switch, then a wire from the relay to the probe and another to the fan. (grounds are important too).
or you can control your fan manually. you can rig up a relay and a switch on the dash to control it.