Missfire, does the fan relay click when you connect the battery?
Does it still run if the relay is removed?
If no to both then most likely the relay contacts welded together.
If the relay does click then something is triggering it, MFTS, Fan after run relay, radiator switch, wiring short, etc.............
If it does still run with the relay removed then the motor feed is shorted to B+.
If the fan has more than one speed, then each speed will probably have a separate relay, Which speed is the fan running at when it shouldn't be?
I'm quite sure that most fan switches will be normally open (ie closed = fan on)
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