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This is a very simple wiring job.

Just get a regular ON/OFF toggle switch and a little bracket to mount it under the dash .
This sw will have 2 terminals..hook a wire to each terminal and run the wires [ I use lamp cord b/c the 2 wires are in the single cord, makes it easy and neat] thru the firewall somewhere and route them over to the switch at the drier . This sw is the one with the 2 pigtail wires on it ..your chassis has two white connectors on the end of this pigtail..just hook one wire to one of the 2 pigtail wires. and the other to the other pigtail wire at these white connectors..that's it, you are done.
This set-up uses all the original low fan circuitry of the stock system [ relay/fuse/resistor/etc], so there is no need to do any crazy elecrtonic additions..just the sw. Meaning it is a very safe , fused system.
When mounting the sw in the cabin, I mount them so when the sw in the ON position, the toggle lever is facing foward, as in the direction of travel of the car..that way you always know the sw is On or OFF from the toggle lever position.. Once the key is OFF, you will get no fan, regardless....
Once you have this additional manual control over your fans operation, Murfey's Law says you will never need it...