The switch on high wouldn't care much, or at all, if it's 12V or 6V.
The lower speed settings, resistances, may be different? But I've used old aftermarket 6V multi speed fan switches on my trucks with 12V without a second thought, and no troubles.


•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-