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Oops, ascii schematic all messed up, try again, dots are spaces, dashes and vertical brackets are connections:
Your mechanic probably added a really jumbo resistor to control speed. What you need to vary the speed from low to high is a switch that controls a relay which selects whether the resistor is in circuit or not.
12V feed from A/C ------------- relay contact 1-
..................|............................ |-relay pole ----- fan
..................|- resistor-- relay contact 2-
12v from control switch, fed by A/C 12v---------- relay coil
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