I've found that electric radiator fans can be had at the junk yard for cheap....like 10 bucks or so.
Not only are they good for the radiator, but you can also use one inside the bus to cool the passengers when parked. They use suprisingly little current. I used to run one inside the old bus to help push the steam out, and i only had telephone wire feeding it.
I've had three of them that i've played with, and all of them would spin backwards when polarity was reversed, this gives you different mounting options.
Quis custodiet ipsos custodes (who will watch the watchmen?)