In My Opinion...............
If you wire directly across the motor it will rev with the motor, however I think it's a bad idea. This is because 1 the spin up time of these fans is not great so will never perform very well, 2 being fed at high frequency from the speedo can't be good for it and 3 and reverse voltage spikes from when you break might pop it as well I would have thought.
What I was on about was that as the negative is switched whatever speedo (brushed) you have the positive is always live so motor positive to battery negative is constant live feed.
I'm happy to be wrong though
