The fan in my car dosen't turn on automatically anymore when the car gets too hot, but it will turn on when i switch on the A/C. So what i had in mind was to place a temperature sensor in the coolant tube just where the thermostat is to measure the temperature and turn on the cars fan with an AtTiny85 and a relay. My question here is: can i leave the cars built in cables to the fan and add some new ones from the AtTiny and the relay, cause i still want the fan to kick in when i turn on the A/C. I'm not quite sure if it will damage the fan cause at some time the car might be hot and therefore the AtTiny will add power to it and the A/C turned on at the same time, that will result in the fan getting two voltages? But it may not damage anything since all the power and ground comes from the same place?