For a taurus fan, you could probably use something in the 35ish range. Thats just a guess though. If he was going to use a relay, it would basically become his power source. . Youd have a constant 12 volt supply, like from the battery on one of the larger posts. On the other one, youd run the power wire to your fan. Then on the two smaller posts, youd run a switched 12v source and the other would be grounded. Its really simple to do and once you actually have it in your hands and begin the install, you'll pretty much know what goes where. Thats assuming you have some idea of what you're doing