You must use a relay to power the fan! Power each hot wire and see which one is high fan speed control. Its probably a fixed magnet, dual windings DC motor. if so, relay the high speed hot wire, to a good 12V source. If you can identify a single high speed power fan wire, isolate the low speed (Provide no power), and use the high speed hot wire. If the ecu output is a ground 0V output for fan ON, wire the relay control input to Ignition +12v (ignition on) and isolate ground from the relay and let the output of the temp control ecu provide ground to the input of the fan relay.
I would verify the output state of the fan control of the ECU before doing any wireing to the ecu. Make sure the control output is either ground (0 Volts = on) or +12v = on. Switching the +12 volts is more conventional, but make sure! If its 12volts = on, provide the ECU output to the relay input, and provide ground to the relay.
Hope this helps, However, I would get a decent wireing diagram before wireing to the ECU. You only get one chance to screw it up!
69 X-44 base coupe, ZZ4, 16lb nodular flywheel, Center force DF clutch + PP, Holley 600DP, Hooker super comp 1 3/4 ceramics, 2.5" dual exhaust w/ X-pipe, flowmaster 40 series mufflers.