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That is exactly what you have..the aux fan circuits had been modified from a standard temp sensor circuit to a Thermistor sensor [ the blu sensor] in 1988.
So, they are not compatable b/c the later one works off the temp input of the ACC panel..You do not have that type on the '86 and the circuitry ..

So, yes..a simple relay and switch type sensor will do the job.
What have you got for a tap hole at the termo housing for mounting a sensor??

Here are a couple of things you may want to know.

Just b/c you have the single fan, you still have two speeds [ that is what the resistor is for].
If you have a/c working, then you are going to want the low fan circuit to also work.
I think they just did not know how these two circuits worked, so they eliminated the Resistor ...

But, I have a schematic for both.

You can also go with a cabin switch as a precaution..many guys do that just to have manual control in traffic, towing, etc.....they just run a jumper wire infrom the a/c high side sw .... so the system is still stock, but you have that over-ride when needed..

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A Dalton