It works by evaporation of water using the same principal as the old wet earthenware food containers and the evaporator cool boxes (fitted in some early campers too). The swamp cooler used ram air to theoretically increase the evaporation rate and to force the cooled air into the vehicle. Although the effect could be measured with a good thermometer in dry conditions, the actual effect is not very apparent when driving and in humid conditions it doesent do anything much at all. Its mostly about the look and retro accessories.