found an image of the standard p51 radiator. as far as thrust goes, i believe this setup did indeed generate enough thrust to cancel out the radiator drag. on early fighters radiators where often placed in locations where the propeller caused undesirable airflow that limited cooling, giving the need for even bigger radiators and thus more drag.(think of planes like the german stuka the hawaker typhoon or the p40... the later famous for the shark teeth painted on the radiator housing) on the p51 the radiator was moved back to clear this area and carefully designed so that the small intake supplied the needed airflow and the heated air provided enough thrust to cancel out the drag so the sum of all there things meant the radiator didn't add to the overall drag, wich made this a very fast, but also economical plane, because good fuel economy meant these planes could escort the bombers deeper over ememy territory
keep in mind that these where masive engines running for hours,generating a lot of heat, and while this heat energy was usually wasted, it energy still came from the fuel. the more efficient an engine will be the less heat there will be generated and the less potential for thrust. but also the less cooling drag will be needed so
i don't think normal car engines and driving conditions will lend itself well to harvesting this effect, but a propperly ducted radiator should improve cooling and reduce drag when done right