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The fan clutch uses right hand threads.
The hi/lo relay looks like this and is located on the relay block under the hood.

 

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My bad, I was talking about the rheostat. Isn’t that what it’s called?

These used to be out in the engine compartment in the side of the fan housing or air conditioning housing.

Or did they change the way this system works?

Enlighten me, I am still old school.

Some models used separate relays for high and low speed fan control, and some (like my 93 burb) are up under the dash as you mentioned.