HA funny you should ask.. i just built the same thing a bout a week ago. I got a 12 volt thermostatically controlled fan switch for 12.00 canadian...it took about two weeks to get to my house. here is the description, copy and paste it into google and you will find the vendor. it works exactly as it says it does and I have ot connected to 4 3" cooling fans for my amp rack...it is in celsuis temperatures only but conversions are easy. I set mine to 30 degrees which is about 86 fahrenheit so when my amps get that hot the fans come on automatically. I have them connected to a relay and the fans only come on when the amps are on.