A 12 volt will just run nice and slow.
I wanted an old six volt dash fan, but frankly, the prices on ebay have gone through the roof for a nice one, as the Bomba crowd likes them.
So, I got a trico vacuum fan. Clamps to the steering column, uses a bit of intake vac, and voila, cool air, and help defrosting that dang rear window.
http://users.erols.com/b-dwheeler/indextrico.htmlModel T and A guys seem to have them.
Down with electrics! Up with vacuum powered everything!!!