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You are already on the website that has instructions on installing a manual switch to turn on your fan. Locate the fan relay, one wire on the fan relay coil is grounded to turn on the fan relay and thus the fan. On my 87 its a green/white stripe. Run a wire from the correct relay coil wire to your spst (single pole single throw) bat handle switch and run a grounded wire to the other terminal on the switch. When you turn on the switch, it will ground the relay coil and turn on the fan if the ignition switch is on.
I wired my primary, and aux fans on the same switch on my 89. Works great in stop and go traffic.
Bought a toggle switch at Pep Boys and hooked both fans up to come on at the same time. One could get a three way and do one or both fans. I live in the Desert and go to Vegas occassionaly. I want to cool down now.
Both the primary and aux fans are controlled by grn/wht wires. I connected both fans with one wire, and the second went to grnd.