I did make a primative ventilation system for my early in-dash Amp. It was a Thermistor (and several resistors to switch the temerature at about 75-80°C) in line with a minature relay (12VDC) to switch a 12VDC fan On/Off. The switching points on the relay are about 9V on and 6V off. This gives you a good hysterisis. The problem I found was It worked on the test bed but in the car the alternator supplies 13-15VDC depending on how much you are revving. So you will need to put zener diode or voltage limiter in the circuit as well