Most radiator fan motors are PM type. So, lets assuming you did hook up your fans as a charger. Lets also say they spin at say 2000 rpm @ 13V (
based off this). That means you have to spin the fan FASTER then that to produce more than 13V, and thus charge your battery. Having the fan behind the radiator isn't going to do that. You'd have to put it on to of your roof or something.