Have a good look though the schematics in your book is my advise.
I'm currently looking for my notes on this little mod. The 'mystery' switch now offers me a manual 'fan on' toggle that buys me time if things are running hot. When I find it I'll post the drawing. Basically it's a ignition switched ckt. that allows you to turn on the rad. fan early and helps a bit on those really hot days (perhaps). This is not a cure if there's a problem with your car but if you know it's hot and you're sittin' in traffic.....you can get the fan running right away.
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