dont forget the amps need to run a car fan i think its 10a to run 1 so if you have a transformer rated 12v 5amp it wont run at full speed and proberly burn out the transformer
im currently building a ventilation with computer fans rated at 12v 0.25amps each fan so my power supply will be 12v 2amp so i can run up to 5 fans without burning my power supply out i will also hook them up to a thermostat set to 21celcuis then the fans will kick in