The canvas bag on the bullbar works with air passing over the wet canvas [water inside but sweats] as the car drives, cooling the water, [like having the fan on] similar principle to a coolgardie meat safe, or an evaporitive cooler.
But if you fill the canvas bag with water, put a sock soaked...
Weaco make a $99 unit plugs in the cig lighter, keeps drinks cool, works at 20-25 deg C below ambient. Console cooler they are called.
http://www.fridgeshop.com.au/coolers.html