Well, if you want to get really technical about it, they radiate, conduct and convect. Automotive cooling systems work through forced convection. Transferring the heat from the engine to the radiator. Where it does, in fact, radiate heat. To increase the efficiency of the radiator, it is placed in the path of an airstream (by movement through said airstream or by means of a fan) to introduce even more forced convection. There is very little conduction, which would be the transfer of heat through matter to the surrounding areas. As in metal to metal contact etc... Yes, air is also considered matter but in heat transfer, matter is usually thought of as a solid. The air moving through the radiator is considered convection since air is considered a fluid.