swamp cooler DIY

Evaporative cooling works by moving air though a fibrous media which absorbs moisture, then gives it up to the air stream. The radiator is just metal fins and it wouldn't hold much moisture, and you would need to properly pump through a distribution header that distributes water over the fins, which the radiator is not designed for. In addition, the radiator doesn't have enough surface area to handle a garage. It is not worth your time.

Your best bet is buy a portable or fixed unit with all components. These things aren't very expensive.